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THE CALIFORNIA SEA HON 

PETERSON AND BARTHOLOMEW 



fic^^VHi^ 



HARVARD UNIVERSITY 




LIBRARY 



OF THE 

Museum of Comparative Zoology 



MUS. COMP. 200L. 
LIBRARY 

JAN 27 lisod 

HARVARD 
UNIVERSITY 



THE NATURAL HISTORY AND 

BEHAVIOR OF THE CALIFORNIA 

SEA LION 



SPECIAL PUBLICATIONS 

This series, published by the American Society of Mammalogists, 
has been estabhshed for papers of monographic scope concerned 
with some aspect of the biology of mammals. 

Correspondence concerning manuscripts to be submitted for 
publication in the series should be addressed to the chairman (and 
editor) of the Committee on Special Publications. 

Copies of special publications may be ordered from the Secretary- 
treasurer of the Society, Dr. Bryan P. Glass, Department of Zoology, 
Oklahoma State University, Stillwater, Oklahoma 74075. 

Price of this issue $3.50 



COMMITTEE ON SPECIAL PUBLICATIONS 

William H. Burt, Chairman and Editor 
Museum of Zoology, 
University of Michigan, 
Ann Arbor, Michigan 48104. 

J. Knox Jones, Jr. 

Museum of Natural History, 
University of Kansas, 
Lawrence, Kansas 66044. 

James N. Layne 

Division of Biological Sciences, 
Cornell University, 
Ithaca, New York 14850. 

M. Raymond Lee 

Museum of Natural History, 
University of Illinois, 
Urbana, Illinois 61803. 




IV 



THE NATURAL HISTORY AND 

BEHAVIOR OF THE 

CALIFORNIA SEA LION 



By 



RICHARD S. PETERSON 

Division of Natural Sciences, 

University of California, Santa Cruz 

AND 

GEORGE A. BARTHOLOMEW 

Department of Zoology 
University of California, Los Angeles 



SPECIAL PUBLICATION NO. I 
THE AMERICAN SOCIETY OF MAMMALOGISTS 

Published December 5, 1967 



!;l 



P^FS 



MUS. COMP. ZOOL. 
LIBRARY 



JAN 27 iS^3 

HARVARD 
UNIVERSITY 



X 



Library of Congress Catalog Card No. 67-29225 
(g) 1967 by The American Society of Mammalogists 



VI 



FOREWORD 

TN 1939 Seth B. Benson, then assistant curator of mammals at the 
-'- Museum of Vertebrate Zoology of the University of California 
in Berkeley, remarked to one of us (G.A.B.) that a natural history 
study of the California sea lion was long overdue. Such a study was 
even longer overdue by the spring of 1965 when the intensive 
phase of our observations on this species was begun. For some 
inexplicable reason, during the more than quarter of a century 
which has elapsed since Benson's remark no detailed field study 
of the behavior of these most accessible of marine mammals has 
been published. 

This neglect is remarkable in view of the wide spread interest 
in marine mammals which has flourished since World War II. Dur- 
ing this period many zoologists, including the authors, have devoted 
much time and effort to field studies of pinnipeds in the Arctic, 
the Antarctic, and on remote islands, but the many thousands of 
sea lions which breed on the islands just off the populous coasts of 
southern California have been neglected. It is remarkable that the 
only extensive publication on the general behavior of Zalophus 
calijornianus is based on the isolated tropical population, Z. c. wol- 
lehaeki, which I. Eibl-Eibesfeldt studied in the Galapagos Islands. 

We hope that our work will help to fill in an anomalous gap in 
the knowledge of pinnipeds, and that it will supply a background 
of natural history against which current and future laboratory and 
experimental studies of California sea lions can be interpreted. The 
present study is obviously little more than a first step toward under- 
standing the social behavior of the California sea lion. Much re- 
mains to be done, particularly the quantification of information and 
the analysis of population dynamics. Nevertheless, even though 
our findings are incomplete, they should be of value as a point of 
departure for subsequent and more sophisticated analyses. 

Our work on Z. calif ornianus included the recording and physical 
analysis of the major patterns of vocalization in this highly vocal 
species, but only the more general aspects of these data are included 
here. The details will be published separately elsewhere. 

As is the case of most field studies a number of persons and insti- 
tutions contributed directly and indirectly to our research. Our 
field work was made possible by a grant (MH-1 1430-01) from the 



National Institutes of Health of the U. S. Public Health Service, 
and by the assistance of the Marine Sciences Division of the Life 
Sciences Department, U. S. Naval Missile Test Center, Point Mugu, 
California. We are grateful for the support of Dr. C. H. Maag and 
Mr. F. G. Wood, Jr., of the Life Sciences Department, and of Cdr. 
H. R. Megrew USN and other officers and men on San Nicolas 
Island. We thank H. J. Chiellon, H. N. Coulombe, and E. C. Beers 
for their assistance in the field, and W. E. Evans and W. Ross for 
generously making field equipment available to us. Mr. C. H. 
Fiscus of the U. S. Fish and Wildlife Service has allowed us to 
include unpublished data from his pelagic studies of sea lions. Part 
of the data analysis and writing was done while one of us (R.S.P.) 
was a National Science Foundation Postdoctoral Fellow at the 
Bureau of Animal Population, Oxford University, and the other of 
us (G.A.B.) was aided by a grant (GB-966) from the National 
Science Foundation. 

Oxford and Los Angeles, 
June, 1966. 



vm 



Contents 



PAGE 

Introduction 1 

Terminology 2 

Methods 2 

General Biology . , 4 

Non-reproductive Behavior 6 

Locomotion 6 

Individual Mannerisms on Land 8 

Special Senses 1 1 

Interspecific Social Relations 14 

Non-reproductive Social Interactions 18 

Reproductive Beha\ ior 2 1 

Male Territories 22 

Behavior of Females 28 

Parturition 31 

Courtship and Copulation 32 

Mother-Young Relations and Ontogeny of Pup Behavior 35 

Perinatal Behavior 35 

Pup Behavior 37 

Mother- Young Recognition and Suckling 40 

Quantifiable Aspects of the Biology of Zalophus 45 

Population Dynamics 45 

Behavior of Individuals 49 

Discussion 58 

Gregariousness 58 

Aggressiveness 59 

Amphibious Adaptations 63 

Evohition of the Social System 66 

Summary 70 

Literature Cited 73 

Index 77 

ix 



Illustrations 

Frontispiece iv 

FIGURE FACING PAGE 

1. Study area on San Nicolas Island 8 

2. Pup marked by hair clipping 9 

3. Female with dye-mark 9 

4. Bow-and-arrow marking technique 16 

5. Female and pup moving from one territory to another _. 16 

6. Prostrate sleeping postures of pups 17 

7. Female sleeping in vertical posture 17 

8. Pup scratching dog-fashion 20 

9. Territorial bull charging man 21 

10. Bull defending area adjacent to dead female 21 

11. Two bulls with territories in water 24 

12. Spatial arrangement of bulls 25 

13. Boundary ceremony of bulls 28 

14. The oblique stare 29 

15. Elevated posture with head-shaking 32 

16. Open-mouth threats by females 33 

17. Neck-rubbing 36 

18. Copulatory posture 36 

19. Pups in shallow water 37 

20. Pups play-fighting 44 

21. Female suckling pup 44 

22. Female suckling yearling 45 

PAGE 

23. Map of San Nicolas Island _,,_ 48 

24. Graph of populations on San Nicolas Island 51 

25. Graph showing changes in populations 53 

26. Graph showing changes in amount of suckling 54 

xi 




INTRODUCTION 

THE California sea lion, Zalo- 
phus calijornianus (Lesson) 
1828, is familiar as a captive ani- 
mal, but its behavior under nat- 
ural conditions has been little 
studied. This is surprising, since 
it is a species which offers un- 
usually favorable opportunities for ethological study; it is highly 
gregarious, relatively sedentary on land, conspicuously sexually 
dimorphic, locally abundant along the coasts of California and 
Baja California, and relatively indifferent to the presence of man. 
The latter characteristic is especially important since it minimizes 
the effect of a himian observer on the social system he is attempt- 
ing to study. 

As Marler (1963:1081) has pointed out, the social behavior of 
pinnipeds is better known than that of any other wild mammals. 
Most of this knowledge, however, is based on three genera— the fur 
seals [CaUorhinus and Arctocephalus) and the elephant seals 
(Mirounga) . Comparative data from other pinnipeds are sorely 
needed, because the behavior of each of the various species is so 
specialized that an order-wide analysis of adequate perspective has 
never been possible (cf. Bertram, 1940) . This paper undertakes to 
provide such information on Zalophus, and to make some prelim- 
inary analyses relating its behavior to that of other pinnipeds and 
to mammals of other orders. 

Only fragmentary data are available on the natural history of 
Zalophus. Scammon (1874) gave a vivid sealer's-eye account of its 
status during the 19th century. Rowley (1929) presented a general 
account of Zalophus behavior (mixed with information on Eume- 
topias) , but the value of his report is reduced by his undocumented 
assumption that the social organization of sea lions and of fur seals 
{Callorhinus) is essentially identical. 

The most extensive study of Zalophus behavior is that of Eibl- 
Eibesfeldt (1955) who spent 60 hours observing the Galapagos sea 
lion (Z. c. luollebaeki) . He described a number of unusual behavior 



2 The California Sea Lion 

patterns which have not been reported in other pinnipeds: paternal 
care of the young (op. cit. :29\) ; bulls' pacification of fighting fe- 
males (op. cit. :292) ; and territorial activity of pups (op. c?7.:294), 
for example. 

The behavior of Zalophus in captivity has recently interested 
several investigators (Schusterman and Feinstein, 1965; Evans and 
Haugen, 1963), but their experimental studies have not yet been 
extended into the field. 

Terminology 

An extensive and picturesqvie terminology developed for pinni- 
peds over the centuries of commercial hunting, and some words 
have become well established in scientific literature (North Pacific 
Fur Seal Commission, 1963) . Several of the more useful of these 
terms are employed in this study. A hull is an adult male, and a 
territorial bull is one defending an area against the intrusion of 
other bulls. Pups are young of the year, especially prior to the 
autumn molt. A rookery is a coastal area occupied by a breeding 
population, and a hauling ground is a terrestrial site occupied by 
non-breeding animals. 

Methods 

The present paper has grown out of a series of studies of the 
population dynamics, distribution, and behavior of pinnipeds on 
the islands in the Eastern Pacific during the past two decades by 
one of us (G.A.B.; see Literature Cited) . However, most of the 
data on the reproductive biology of Zalophus were obtained during 
three and a half months of intensive work on San Nicolas Island, 
California, during the late spring and summer of 1965. San Nicolas, 
the outermost of the California Channel Islands, is about 66 nautical 
miles southwest of Los Angeles Harbor. (For information about 
San Nicolas and the other islands in this group, see Hillinger, 1958, 
and Winslow, 1960) . 

Observation.— The primary method of study was observation 
from a blind on a bluff overlooking a rookery on the south shore of 
San Nicolas Island (Fig. 1) . The behavior of individual sea lions 
and their social interactions were recorded in detail in a notebook 
at the time they were observed. Characteristic activities were re- 



Introduction 3 

corded with both still and motion picture photography and vocaliza- 
tions were recorded on a Uher 400 Report-S tape recorder. Censuses, 
including absolute numbers of all classes of individuals and the lo- 
cation and activity of marked individuals, were made on the study 
rookery nearly every day between 4 June and 16 September 1965. 
The other sea lion rookeries on the island were visited every few 
days. Observations were scheduled to sample all hours, day and 
night. During the hours of daylight we watched the animals through 
binoculars and a spotting telescope so that we were able to keep a 
large area, not only that immediately adjacent to our blind, under 
surveillance. Our first nocturnal observations were made with an 
infra-red "sniperscope," but we soon found that it was possible to 
crawl in among the animals at night and watch them at close range 
by available light. This latter method was much more instructive. 
We recorded nocturnal observations on a portable tape recorder, 
speaking quietly to avoid disturbing the animals. 

Marking.— Thirty-nine pups were marked by clipping out letters 
and numbers in the hair of their backs. Pups do not retreat into the 
sea when approached, and it was a relatively simple matter to hold 
them down and clip the patterns in their hair with a pair of scissors 
(Fig. 2) . The clipped areas were paler than the rest of the pelage 
and the markings persisted for about three months. Eleven adult fe- 
males were marked with a black aniline dye (Nyanzol D, Nyanza 
Chemical Co.) dissolved in hydrogen peroxide. The dye solution 
was placed in a projectile syringe modified by removing the needle 
and drilling holes around the distal end. The proximal end, con- 
taining a plunger and small explosive charge, was fitted to the head 
of an arrow (see Short and King, 1964) , The arrows were then 
shot at selected female sea lions with a bow, and upon impact the 
dye sprayed out and made dark, irregularly shaped blotches which 
were still recognizable on the light brown pelage of the females after 
more than three months (Fig. 3) . When struck by the arrow the 
females usually did not flee, but merely reared up and peered about 
as if trying to locate the source of the disturbance. This method 
allowed us to select specific individuals and mark them without 
disturbing the other animals in the rookery. 

Zalophus bulls are dark brown, and the black dye proved un- 
satisfactory for marking them, particularly when they were wet. 
However, most of the territorial bulls bore conspicuous identifying 



4 Tlie California Sea Lion 

scars, and we were able to identify and follow 14 of them at the 
study area by their patterns of scars and wounds. 

Experiments in immobilizing bulls with succinylcholine chloride 
were undertaken as a possible aid in marking. The drug was in- 
jected through an avuomatic projectile syringe fastened to an 
arrow as described above (Fig. 4) . Seven bulls were experimented 
on in areas remote from the study area. The paralytic dose varied 
from 70 to 90 milligrams. The estimated weights of these bulls 
ranged from 175 to 275 kg, thus, the dosage was approximately 0.25 
to 0.50 mg/kg, very similar to that required for immobilizing Callo- 
rhinus bulls (Peterson, 1965a: 692) . 

General Biology 

Zalophus is a medium-sized otariid of marked sexual dimorphism. 
Adult males weigh 200-300 kg and are 200-250 cm in length, while 
adult females weigh 50-100 kg and measure 150-200 cm. Thus, 
adult Zalophus fall in the general size range of Callorhiniis (Scheffer, 
1958:141-142), but the difference between the male and the female 
is somewhat less extreme in Zalophus than in Callorhiniis. 

Three geographically separated populations of Z. californianus 
are recognized by Scheffer (1958) as subspecies: Z. c. wollebaeki in 
the Galapagos Archipelago, Z. c. japonicus in Japanese waters, and 
Z. c. californianus on the west coast of North America (see also King, 
1961:218) . The morphologic differences between the three popula- 
tions are relatively minor. 

Z. c. californianus, on which the present study is based, breeds on 
islands in the Gulf of California and along the west coast of Mexico 
south to Mazatlan, and off the Baja California and California 
coasts northward to San Miguel, the most northwesterly of the 
California Channel Islands. Non-breeding aggregations of the spe- 
cies occur regularly over a much wider area of the eastern Pacific, 
ranging as far north as British Columbia. 

A conspicuous, although not complete, sexual segregation occurs 
in Z. c. californianus during the non-breeding season. The adult 
and sub-adult males generally move northward as soon as the sum- 
mer breeding season ends, and return to the rookeries in spring. 
The females and young either remain in the vicinity of the breeding 
rookeries the year round, or, as in the case of some members of the 



Introduction 5 

population in the California Channel Islands, apparently move 
southward in winter. As a result of the differing seasonal move- 
ments of males and females, during the winter there are many fe- 
males but few males in the southern half of the range of the sub- 
species while many males occur north of the present breeding 
range (Fry, 1939; Bartholomew and Hubbs, 1952; Bartholomew and 
Boolootian, 1960; Orr and Poulter, 1965) . The seasonal segrega- 
tion and migration in Zalophus, however, are not nearly so well 
defined as that of Callorhinus (Kenyon and Wilke, 1953) . 

The food of Zalophus consists primarily of squid, including Lo- 
ligo, and small fish such as Engraulis and Merluccius (Bonnot, 
1928; Scheffer and Neff, 1948; Fiscus and Baines, 1966) . From ship- 
board we have seen Zalophus that appeared to be feeding during 
both day and night, but it is not known if they have a daily feeding 
cycle as do Callorhinus (Fiscus et al., 1964:36) and Leptonychotes 
(M. S. R. Smith, 1965:15). 




NON- REPROD UCTIVE BEHA VIOR 



Locomotion 

IN Water.— The swimming of Zalophus re- 
sembles that of other otariids (see Howell, 
1930, and Ray, 1963) . The main propulsive 
force is supplied by simultaneous inward and 
downward strokes of the foreflippers, while 
the hindflippers function primarily as stabi- 
lizers or as rudders, depending upon their plane 
of rotation. When turning horizontally, sea 
lions extend their foreflippers and bank like a turning bird. 

Contrary to statements of Rowley (1929:19) , sea lions frequently 
"porpoise" while swimming; that is, they leap from the water in a 
shallow arc and re-enter head first. This pattern of behavior is 
most common during sustained, rapid swimming. Groups of 5 to 20 
young individuals sometimes swim in single file, porpoising one 
after another. An intruder being chased at top speed by a territorial 
bull usually porpoises while fleeing from its attacker. Young animals 
often leap exuberantly out of the water during play and may 
employ porpoising in play chases. 

Zalophus use waves to assist them in coming ashore and going 
to sea. They are skillful "body-surfers" and often glide down the 
front of a wave, just submerged or partly above the surface. After 
the wave breaks they ride it up to the beach, keeping their heads 
and sometimes shoulders above the foam. Animals coming ashore 
on a level beach usually take advantage of incoming waves, and 
they ride the backwash when departing. On a steep, rocky shore, 
they often utilize a wave to assist in vertical movement and may 
leap from the top of the wave to land on a ledge which could other- 
wise be reached only after much climbing. 

Aggregations of sea lions commonly sleep in the water, often in 
protected areas near the shore. Sleeping animals frequently lie on 
one side and elevate a foreflipper or hindflipper above the water 
as described by Eibl-Eibesfeldt (1955:299), but we have never seen 
them assume the "jug" posture of fur seals, that is, floating on one 



Non-reproductive Behavior 7 

side with the hindfhppers and one forefhpper pressed together and 
held in an arch above the water. Zalophus is essentially a coastal 
animal that hauls out on shore frequently throughout the year, and 
as such may not need to be so well adapted to sleeping in the water 
as a pinniped like Callorhinus which remains at sea for months at 
a time. Furthermore, Zalophus may be less bouyant than the fur 
seals (which have thicker pelage) ; C. H. Fiscus, who has studied 
seals in the pelagic environment, reports (pers. comm.) that both 
Eiimetopias and Zalophus are much more difficult to collect at sea 
than fur seals because the sea lions usually sink when they are shot. 

On 'La^^d.— Zalophus, like other otariids, but unlike phocids, are 
capable of quite effective locomotion on land. In difficult terrain, 
such as among slippery rocks, an aggressive bull can probably move 
more rapidly than a man. Top speed across smooth rocks probably 
approaches 15 miles per hour, but this can be maintained for a few 
meters only. 

Terrestrial locomotion is extremely variable, but several basic 
patterns can be discerned. These may be used simultaneously and 
in various combinations. The points of traction are the whole 
manus of the foreflipper and posterior portion (heel) of the hind- 
flipper. When moving slowly, the animals walk, using all four 
limbs alternately, much like a terrestrial quadruped. However, the 
hind limbs are incapable of moving independently more than a 
short distance and so the steps are quite short. As the animal walks, 
the fore and hind limbs are swung laterally as well as forward, and 
barely clear the ground. With each step the shoulders and hips are 
rotated strongly, and the head and neck swing sinuously from side 
to side (Fig. 5) . Sometimes, when moving in a leisurely manner, a 
sea lion will flex the posterior half of its vertebral column laterally 
with each step so that the hindquarters are displaced to one side 
with every stride. Some individuals walking in this manner seem 
always to swing the hindquarters to the left, others always to the 
right; in a group of non-territorial bulls, we once counted 16 swing- 
ing to the left and 11 to the right. 

When moving more rapidly, sea lions gallop, using the heels of 
both hindflippers as a single pivot while the foreflippers move 
forward slightly out of phase with each other. A galloping animal's 
head does not swing laterally, but bobs up and down vertically. 



8 The California Sea Lion 

Galloping pups move their foreflippers simultaneously in a rapid, 
rotary motion. 

While moving over smooth sand, going downhill, or in a few 
inches of water, Zalophus often stride with long steps of the fore- 
limbs, and drag the hindquarters limply along behind, sometimes 
with the hindflippers elevated slightly so that the weight rests on 
the abdomen. This striding pattern permits very rapid movement 
across wet flats or in the wash of the surf, and frequently is em- 
ployed by territorial males when rushing at intruders. 

An intruding bull or a submissive, estrous female will sometimes 
crawl along slowly on its belly by alternate lateral movements of 
its flippers, keeping the head and neck close to the ground. This 
crawling is somewhat like the "sinuous" locomotion of Lohodon car- 
cinophagus described by O'Gorman (1963:847) . 

Individual Mannerisms on Land 

Sleeping Postures.— Sea lions have extremely flexible bodies and 
assume a wide variety of resting and sleeping postures while on land. 
Several of these are quite stereotyped and are regularly employed 
by individuals of both sexes and all ages. In one characteristic 
sleeping posture the animal lies prostrate on its belly and folds all 
four flippers underneath. In another, the animal lies on its side with 
the foreflippers against the body and the hindflippers appressed 
and extended to the rear. Zalophus also sleep sitting up on the fore- 
flippers, with the head thrown back and the nose pointing vertically, 
evidently balancing themselves comfortably in this way (Figs. 6 
and 7) . 

Grooming.— Despite their short sparse hair (Scheffer, 1964:293), 
Zalophus spend much time grooming, just as do marine mammals 
such as Callorhinus and Enhydra, which have long dense pelage. 
The most conspicuous and frequent grooming mannerism of Zalo- 
phus is a dog-like scratching with one of the hindflippers. By bend- 
ing the body laterally, a sea lion can groom nearly any part of the 
anterior two-thirds of the body by this means. The digits of the 
flipper being used are flexed so that the terminal flaps fold down 
and the three functional claws are exposed (Fig. 8) . Another com- 
mon pattern of grooming involves rubbing with one foreflipper 
while balancing on the other; the head and neck are usually 




Fig. 1. Study area on southwest shore of San Nicolas Island. See Fig. 23 for 
precise location. 





Fig. 2. Pup that has been marked by hair clipping; grooming itself by scratch- 
ing dog-fashion with hindflipper (top) . 

Fig. 3. Female with dye-mark on flank (bottom) . 



N on-reproductive Behavior 9 

stretched upward and the palmar surface of the manus, particularly 
near its leading edge, is rubbed back and forth across the body. 
Almost the entire posterior half of the body can be reached this way. 
The foreflipper can also be rotated forward and used to rub the 
head when the neck is bent laterally. Less frequently, Zalophus 
rub their snouts across their hindquarters, and nuzzle or nibble 
their pelage with their incisors as dogs do. 

Individuals of both sexes and all ages rub their bodies against 
rocks or against other animals, similar to ungulates at a rubbing post. 
They may also lie flat on the sand or on a smooth rock and rub 
their bellies or backs with lateral flexures of the body. Another 
characteristic grooming mannerism consists of rubbing the vibrissae 
against a smooth rock with a firm stropping motion, first on one 
side of the snout and then the other. Such "whisker-stropping" is 
frequently repeated by adults just after they have hauled out of 
the water. 

Excretion.— Unlike most terrestrial mammals, Zalophus assume 
no special postures when urinating or defecating, nor do they pay 
any attention to voided wastes. They may defecate or urinate when 
lying prostrate or while moving about on the rookery. The fre- 
quency of defecation is good evidence that both the territorial bulls 
and nursing females feed regularly during the course of the breed- 
ing season. 

Zalophus, particularly nursing females, sometimes vomit a white 
milky fluid and emit a characteristic coughing noise in the process. 
This regurgitation is accompanied by violent contractions of the 
thorax and by lateral shaking of the head; it appears to be labored 
and difficult. 

Drinking.— Although field observations on the drinking of sea 
water are always hard to verify, we saw several instances that warrant 
description. Territorial bulls immersed their snouts in pools of 
sea water and performed gulping motions, interspersed with chew- 
ing movements of the jaws after the head was lifted from the water. 
We do not know if water was ingested, or whether this behavior was 
merely mouth-wetting. We have both observed similar behavior 
among bulls of Callorhinus on warm days. 

Responses to Weather.— Although Zalophus breed successfully 
in the tropics (in the Galapagos Islands, for example) , individuals 
appear to be sensitive to high air temperatures and direct solar 



10 The California Sea Lion 

radiation. On San Nicolas Island the sky is covered with a low, 
unbroken overcast at least 25 per cent of the time during the 
summer (Winslow, 1960:30) . Whenever the sky clears and direct 
sunlight strikes the rookeries, those sea lions not already at the 
water's edge arouse and move onto wet sand or into shallow water. 
Bulls appear to be especially sensitive, and during sunny days most 
of them keep thoroughly wet by frequent visits to the water. 
Females also go to the water's edge frequently during the day. 
The pups, however, are much less responsive to heat. A female 
that is still defensive of her new pup may drag it with her when 
she visits the edge of the sea or a tide pool, probably because she 
needs to cool off and is unwilling to be separated from her pup, 
not because the pup needs protection from heat. 

On bright, clear days, sea lions sleeping in a prostrate position 
often extend their hind- and foreflippers upward and hold them 
motionless. By analogy with the more elaborate flipper-waving 
of Callorhinus (Bartholomew and Wilke, 1956:331, and Irving 
et al., 1962:282), this behavior could serve to dissipate heat. But 
in Zalophiis, these postures are sometimes seen on cool, overcast days, 
and pups also use them. Thus, we cannot be certain that a thermo- 
regulatory function is served. The habit of staying damp or resting 
on damp sand whenever heat stress exists may be sufficient in Zalo- 
phiis to unload excessive heat. It may be significant that we have 
never seen behavior that we can classify with certainty as panting, 
even by bulls engaged in vigorous fighting. 

Paulian (1964:28) suggests that the distribution of Arctocephalus 
on the windward side of He Amsterdam may be related to this fur 
seal's need for cooling. It is possible that the distribution of Zalo- 
phus on the Channel Islands, including San Nicolas, may be simi- 
larly related to wind direction. The great majority of the strong, 
steady winds on these islands are from the west and northwest 
(Winslow, 1960:29), and almost all the rookeries are on beaches 
with western exposures. 

Strong winds have little effect on Zalophiis, but if the surf is 
heavy the animals move inland above the breakers. Stinging sand 
storms occur frequently on the windward side of San Nicolas, but 
the animals seem indifferent to the pelting sand and allow it to 
drift around their bodies. 




Non-reproductive Behavior 11 

Special Senses 

Vision.— The visual acuity of Zalophiis in air 
is apparently limited. Even in broad daylight we 
were able to approach within two meters or less 
of aggregations of females and pups so long as we 
crawled slowly and kept our silhouettes low. How- 
ever, a man in full view walking toward sea lions 
will alarm them at 75 meters or more during the 
day, and at 25 meters on a clear night. 

When a strange object is near them (a crawling 
man, for example) sea lions extend their necks toward it and peer 
intently, swinging their lowered heads from side to side as if trying 
to see it better. Once when we were lying prostrate on the open 
beach at midday recording sea lion vocalizations, a female ap- 
proached the canvas-covered tape recorder which was lying beside 
one of us, peered at it for about ten seconds, sniffed it, and then 
settled to sleep about a meter away. When the recorder clicked 
softly as we threw a switch, however, the animal bolted away. The 
sight of a man with a parka over his head, a microphone in his 
hand, and a recorder beside him had not been enough to cause 
alarm, but a simple clicking sound caused a strong flight reaction. 
We have had full-grown bulls actually brush against us in broad 
daylight while we were sitting fully exposed but with our outlines 
obscured by a boulder against which we were leaning. 

As discussed later, Zalophiis are as active at night as during the 
day, but their visual discrimination at night is even poorer than 
during the day. At night, by remaining close to a cliff so that we 
were not outlined against the sky, we were able to crawl into the 
aggregations of females and pups and remain for as long as we 
wished, so long as we did not allow the animals to sniff our faces. 

However, despite their apparently poor aerial vision, sea lions 
readily discriminate bold outlines and rapid movements. Any 
bulky vertical object rising above the general level of the females' 
heads can cause an alarm response, especially if it moves. A vertical 
image of this sort is presented by a threatening territorial bull 
standing with elevated head and neck, as well as by a man standing 
up. We frequently saw aggregations of sea lions take alarm at a 
threatening, silhouetted bull, exactly as they would at a man ap- 
proaching on foot. 



12 The California Sea Lion 

The visual signals utilized by sea lions in social interactions are 
simple and obvious and require little more than perception o£ size 
and movement. An example is the boundary ceremony of territorial 
bulls, in which the head is held high and swung slowly from side 
to side. 

Hamilton (1934:294) has described behavior in Otaria indicative 
of limited visual acuity and he suggests without further documenta- 
tion that this may be related to hyper-sensitivity in bright light 
or to irritation of the eyes by constant wind. On the basis of ocular 
morphology it has been suggested that the eyes of pinnipeds are 
adapted for acute vision in air as well as in water (Walls, 1942:446) . 
Although Zalophus may employ relatively precise visual discrimina- 
tion when feeding under water (Schusterman et al., 1965; Hobson, 
1966) , it appears that their visual discrimination in air is of a low 
order. Judging by their reactions to man, some pinnipeds such as 
harbor seals {Phoca vitulina) see much better in air than do sea 
lions. 

Hearing.— The California sea lion is among the most vocal of 
mammals; a wide variety of vocal signals are continuously being 
exchanged between adults. As discussed later, the mutual recogni- 
tion of mother and pup appear to depend primarily upon identifi- 
cation of individual vocalizations. These facts are circumstantial 
evidence for good hearing ability, yet Zalophus are relatively un- 
responsive to sounds such as those of aircraft or even the low conver- 
sation of humans only one to two meters distant. Sharp, sudden 
sounds cause immediate alarm, however, and every animal in a 
noisy aggregation will rise up in alarm if two rocks are struck to- 
gether. They seem habituated to loud, general noises such as that 
of hundreds of surrounding animals and the crashing of the surf, 
while simultaneously discriminating and reacting to particular 
vocalizations of nearby individuals. 

Zalophus may possess the ability to echolocate underwater (see 
Evans and Haugen, 1963; Poulter, 1963; Schusterman and Fein- 
stein, 1965) ; this would be additional reason to suspect an acute 
auditory sense. From the importance of vocal signaling in their 
behavior on land, it seems certain that the sense is highly developed. 

Chemical Senses.— Olfactory clues apparently are employed by 
Zalophus in at least two social situations on land: the interactions 
between males and estrous females, and in mother-pup recognition. 



Non-reproductive Behavior 13 

In the latter situation, at least, a high level of olfactory discrimina- 
tion appears to be involved. 

Bulls of Zalophus have a slight musky odor, but it is not nearly as 
strong as that of Callorhinus. We saw nothing to suggest that 
bulls of Zalophus use scent to mark their territories as has been 
postulated for Callorhinus (Peterson, 1965&:81) . However, the 
rocks and sand of the rookeries are extremely odoriferous, as we 
found when crawling among the animals. It is not difficult to imag- 
ine that these smells, mostly resulting from urine and feces, could be 
used to recognize established hauling-out places, even when there 
were no other animals on shore. 

Several instances of the apparent use of olfaction occurred after 
the carcass of a dead pup fell into a small, stagnant pool where 
adults had been wetting themselves frequently on warm days. On 
9 June 1965, we watched a succession of bulls and females approach 
this pool as if about to immerse themselves in it, only to move away 
after sniffing at the floating carcass. 

Like Otaria (Hamilton, 1934:295) and Callorhinus (authors' 
observations) , Zalophus may be able at times to detect the smell of 
humans. On 11 June 1965, in the company of four seamen who 
were to assist us in marking, we approached the study area from 
upwind and remained completely out of sight of the sea lions. Never- 
theless the entire rookery took alarm while we were more than 100 
yards distant, out of sight behind a hill. It is possible that the ani- 
mals detected the odor of our sweating bodies, yet during our ex- 
cursions among the animals at night individual sea lions seldom 
took alarm until they sniffed directly and carefully at our faces. 
It may be that these animals are capable of discriminating various 
odors, and react selectively to especially strange or strong ones. 

We have not seen any behavior which suggests that sea lions use 
their gustatory sense while on land. During hundreds of hours of 
observation we have never seen female Zalophus lick their newborn 
pups {contra Eibl-Eibesfeldt, 1955:300) and it appears improbable 
that taste functions in mother-young recognition in the population 
we studied. King (1964:124) reported that the tongue of Zalophus 
has a poor supply of taste buds. 

Tactile Sense.— The frequency of social nuzzling and nose-rubbing 
in Zalophus suggests that the long vibrissae are highly developed 
sensors (Fig. 10) . Vibrissal touching occurs in almost every close 



14 The California Sea Lion 

interaction between pairs or groups of sea lions: between females 
and pups, bulls and females, pairs of threatening bulls, pairs of 
threatening females, and among young non-breeders. The vibris- 
sae are extremely movable and can be rotated from the folded- 
back resting position until they extend almost directly forward 
from the muzzle. We found that when curious sea lions sniffed 
at our faces at night, they extended their vibrissae forward and 
gently brushed them against us, while repeatedly opening their 
nostrils. 

Except in the breeding season, Zalophus are strongly thigmotactic, 
and usually lie in closely packed groups, occasionally even on top 
of each other. At times, however, they become quite reactive to any 
light touch; females with newborn pups, for example, react very 
aggressively to even the lightest contact from another adult or pup. 
Thus, it appears that the skin of Zalophus may be quite sensitive to 
touch (as is that of Monachus, Kenyon and Rice, 1959:234), but 
that responses to tactile stimuli, as with those to visual, acoustic, 
and olfactory stimuli, are selective. 

Interspecific Social Relations 

The rookeries and hauling grounds of Zalophus are regularly 
visited by scavengers and are also utilized by other marine verte- 
brates for breeding and resting. Consequently, there are frequent 
opportunities for observing the relations between Zalophus and 
other species. The other pinnipeds which commonly occur in the 
California Channel Islands, Mirounga angustirostris, Phoca vitulina, 
and Eumetopias jubata, frequently share beaches with Zalophus, 
but no two of the species breed during the same season at the same 
location. Brandt's cormorants (Phalacrocorax penicillatus) some- 
times breed on the same beaches with Zalophus, and western gulls 
(Larus occidentalis) breed immediately inland of the sea lion rook- 
eries. The western gulls, as well as ravens (Corvus corax) , and 
Channel Island foxes [Urocyon littoralis) scavenge among the sea 
lions. 

Elephant Seals.— On San Nicolas Island only a few elephant 
seals come ashore during the sea lions' breeding season. The ex- 
tensive and sustained interactions of Mirounga and Zalophus have 
been described elsewhere (Bartholomew, 1952:380-382), and we 



Non-reproductive Behavior 15 

have little to add here. In general, the interspecific relations of 
these two species resemble the intraspecific behavior of sea lions 
under similar circumstances; that is, there is a weak dominance 
relationship among the non-breeders based upon individual size, 
and strong territoriality among breeding bulls and parturient fe- 
males. Territorial bulls of Zalophus threaten and chase elephant 
seals that come too close, just as they would any other large animal 
that approached, and even female Zalophus may react aggressively 
toward small elephant seals. On 5 June 1965, we watched a female 
Zalophus, still vigorously defending her newborn pup, threaten a 
young elephant seal that was moving toward her. The elephant seal 
reared up briefly as the female approached, then changed its course 
and moved aside. 

Harbor Seals.— Aggregations of Phoca frequently haul out on 
active Zalophus rookeries, but harbor seals tend to remain in com- 
pact groups and do not intermingle freely with the sea lions. Sea 
lion bulls sometimes threaten individual harbor seals in the water 
near shore, and chase them briefly. On land the harbor seals are 
treated with almost complete indifference, as though they were 
sea lion pups or submissive juveniles. On 20 June 1965, territorial 
bull Zalophus were defending territories on San Nicolas area 3B 
where throughout the summer aggregations of harbor seals persisted. 
Whenever one of the bulls approached, the aggregations of Phoca 
would quickly part to let him pass through, much as a pod of sea 
lion pups would do. Individual sea lions and harbor seals usually 
avoid contact with each other; we never saw Zalophus crawling 
over Phoca as they do across each other or across Mirounga. But 
since harbor seals move more slowly than sea lions on land, individ- 
uals of the two genera sometimes meet. When this happens, the 
harbor seal usually rolls on its side, keeps its head lifted upright, 
snaps its jaws at the approaching sea lion, and simultaneously makes 
a rapid waving or scratching motion in the air with the uppermost 
foreflipper. We never saw a harbor seal actually bite or scratch a 
sea lion, but on one occasion we saw a female Zalophus with a new 
pup viciously chase a harbor seal that had landed quietly nearby. 
She dashed toward it and made a few jabbing nips at its back; the 
harbor seal quickly abandoned its threatening posture and scurried 
rapidly away. 



16 The California Sea Lion 

Harbor seals are readily alarmed and appear to have much more 
acute vision than sea lions. It is, therefore, difficult for a man to 
approach a mixed aggregation of sea lions and harbor seals, since 
the harbor seals visually see him and dash for the water, and then 
the sea lions react by joining the stampede. The converse does not 
hold, for harbor seals do not react to the frequent, self-induced 
panics of young sea lions playing among them. When the sea lions 
make such a rush toward the water, the harbor seals raise their 
heads and look about alertly, but settle to sleep again if no danger 
is sighted. 

Steller Sea Lions.— The breeding ranges of Zalophus and Eume- 
topias overlap at the present time only on San Miguel Island (Bar- 
tholomew and Boolootian, 1960:373), and the breeding season of 
Eumetopias occurs before that of Zalophus. Mixed non-breeding 
aggregations of the two genera are common north of the breeding 
range of Zalophus and the interactions in these groups have been 
described by Orr (1965a: 165). The interspecific relations are gen- 
erally parallel to those which occur between Mirounga and Zalo- 
phus, in that the animals tend to interact in the usual intraspecific 
manner. 

Gulls.— Western gulls were continuously present on the sea lion 
rookeries, even at night. Breeding colonies of the gulls occurred 
within 50 meters or less of the sea lion rookeries at San Nicolas Area 
1. The gulls are very bold and walk about among the sea lions 
quite freely, sometimes even alighting on them. They are particu- 
larly responsive to fresh placentas, and each birth attracts a noisy 
flock of gulls almost immediately. The gulls also eat Zalophus feces. 
They sometimes peck at the anus of a pup, following it about the 
rookery in their attempts. 

For the most part, sea lions pay no attention to the scavenging 
gulls, but occasionally a female reacts to a nearby gull with an 
open-mouth threat. Pups and juvenile animals sometimes snap at 
the gulls and chase them briefly. Very young pups occasionally 
appear to be frightened of the gulls. 

Sea lions are sensitive to sudden movements and sharp noises. 
Thus, any excitement among the gulls, or their alarm calls, may 
cause a widespread reaction among the sea lions. A single alarm 
call from a gull, for example, can cause one sea lion to dash toward 
the water, and this movement may set off a general stampede that 






^Jl?^ 




-4 



' .--^ 



-iSi' 




^^^!H 



^ '^^Ai^^U^^Au^U 



Fig. 4. Bow-and-arrow technique used for marking and narcotizing animals 
(top) . See text for details. 

Fig. 5. Female and pup moving from one territory to another (bottom) . 
Note strong lateral movement of head and neck as she walks. Pup in foreground 
has responded to vocalizations that female directed at her own pup and is mov- 
ing toward the pair. 






Fig. 6. Prostrate sleeping postures of pups (top) . 

Fig. 7. Female sleeping in characteristic vertical posture in a semi-aquatic 
territory (bottom) . Note territorial male in water to right of females. 



Non-reproductive Behavior 17 

will empty the beach of animals. Sometimes the birds may hover 
and squawk near an aggregation where there is a fresh placenta, 
almost as though intentionally trying to frighten the sea lions away. 
But the sea lions rarely react strongly to this situation. They ap- 
pear to be relatively unresponsive to disturbances in the breeding 
rookeries during the breeding season. 

Man.— Although one can closely approach sea lions with only the 
most rudimentary sort of stalking, this is apparently owing to the 
sea lions' poor aerial vision rather than indifference to the presence 
of man. On the California Channel Islands, the usual reaction of 
sea lions as soon as they become aware of the approach of humans 
is a wild stampede into the water. This reaction is not invariable 
in the species, however. On the Galapagos Islands, individuals of 
the subspecies Z. c. wollehaeki share the indifference to man which 
is so conspicuous of most vertebrates of oceanic islands. At Punta 
Espinosa on Narborough Island in the Galapagos, some of the 
Zalophus were so unafraid of man that one of us (G.A.B.) was able 
to walk directly up to and touch adult females, and on one occasion 
was able to play tug-of-war for several minutes with a juvenile male 
which held a piece of driftwood in its jaws like a dog with a stick 
(see also Eibl-Eibesfeldt, 1955:298) . 

During the breeding season, the alarm reaction of the Zalophus 
in the Channel Islands population changes markedly. The with- 
drawal of an aggregation at the approach of a man becomes rela- 
tively slow, almost reluctant. In June and July, a territorial bull 
may refuse to retreat and may chase a man to its territorial bound- 
ary (Fig. 9) . On 3 July 1965, we found one bull alone on a beach 
with the carcass of a female; this bull reacted aggressively when we 
approached, and refused to retreat (Fig. 10) . 

Similarly, females with new pups are not apt to flee from a man 
and will remain on the beach with their pups long after the other 
adults have dashed into the water. Occasionally, they even rush 
threateningly at a man. 

Chronic human disturbance causes Zalophus to abandon a rook- 
ery temporarily. Our repeated efforts to mark some animals at the 
beginning of the study caused many females and pups to move, over 
a period of two days, to a new site 30 to 50 meters away. Over a 
period of several weeks they gradually drifted back and reoccupied 
the old site. If rookery disturbance persists for years, as has hap- 




18 The California Sea Lion 

pened at part of San Nicolas Area 1 because of military operations, 
the site is finally abandoned entirely. 

Zalophus are much less fearfvil of man in the water than on land, 
and they often approach swimmers and small boats closely. Groups 
of 15-30 young individuals (up to 3 years old) frequently rush to 
meet a boat and may accompany it for 100 meters or more showing 
every sign of excited curiosity. 

Non-reproductive Social Interactions 

During the non-breeding season (ap- 
proximately August through April) , the 
aggregations of sea lions on land have no 
stable social organization. The same is 
also true on the hauling grounds (as con- 
trasted with the rookeries) throughout 
the breeding season. However, certain 
stereotyped interactions can be seen in these unorganized groups, 
and the behavior follows the general patterns described in this 
section. 

Dominance Relations.— There is a clear, size-related dominance 
relationship among non-breeders, regardless of sex. When a small 
individual is approached or threatened by a larger one, the smaller 
usually yields by scrambling aside or running, but there is no sus- 
tained aggressive behavior during these encounters. The hierarchi- 
cal relations are weak and transient, but readily apparent in any 
non-breeding aggregation. 

Gregariousness.— Non-breeding individuals are always highly gre- 
garious while on land. They often pack themselves so closely to- 
gether that arriving or departing animals must crawl over the bodies 
of the others. These congested groups form even though large sec- 
tions of adjacent beach remain unoccupied. The animals are 
strongly thigmotactic and while sleeping tend to keep their bodies 
pressed against each other. Even non-territorial bulls lie close to- 
gether in this way, especially in winter. Sometimes small individ- 
uals may lie on top of larger ones, but this is not tolerated for ex- 
tended periods of time. They also rest on the backs of elephant 
seals or on piles of kelp, seemingly preferring these soft resting 
places to rocks. 



Non-reproductive Behavior 19 

Play.— It is a common observation that sea lions spend much time 
and energy at play, both on land and at sea (see, for example, 
Eibl-Eibesfeldt, 1955:297) . Play is difficult behavior to categorize 
and describe (Loizos, 1966) . In sea lions, as in other mammals, 
play is most frequent among non-breeders, especially juveniles. 
The activity of pups will be described separately. 

The most common pattern of play involves swimming about in 
the surf. An animal may ride down the front of a wave, usually 
just beneath the surface, until it breaks, then veer sharply to one 
side inside the wave, or even leap out over the crest, and then swim 
back out to await another wave. Sometimes several animals ride 
one wave together, and they may tumble over each other in the 
breakers. Frequently they chase each other in the water, porpois- 
ing and turning sharply. Often three or four juveniles engage 
in play contests over occupancy of a boulder or ledge in shallow 
water. One animal will climb onto the boulder and try to remain 
there as the others climb out of the water and try to displace it by 
shoving and pushing. Such contests may continue for a quarter of 
an hour. 

Subadult males (probably sexually mature but not yet full 
grown) sometimes threaten each other almost as though they were 
on territories, using the same postures and movements as the terri- 
torial bulls. Non-breeding bulls also behave in this manner. They 
chase after each other, bark continually, stand chest-to-chest, push 
each other, shake their heads from side to side, and exchange open- 
mouthed threats, all of which appear to be intermediate between 
the play of young pups and the more aggressive interactions of terri- 
torial bulls. Wounds are not inflicted, and groups, rather than 
pairs, frequently participate in the skirmishes. Territorial bound- 
aries are not maintained. For discussion of this behavior see sec- 
tion on ontogeny of aggressive behavior. 

Alarm Reactions.— On the California Channel Islands, the usual 
alarm reaction of non-breeding Zalophus consists of a frantic dash 
for the water, even if this involves leaping off a high cliff onto a 
rocky beach below. The alarm is highly contagious, and usually 
spreads rapidly through an aggregation. If the reaction is of low 
intensity several animals will simultaneously rear up and look 
about and then gradually retreat from the direction of the disturb- 
ance, or if the disturbance is more obvious they will rush to the 



20 The California Sea Lion 

water's edge before stopping to look about. Sometimes hundreds 
of animals stampede into the water without pausing to identify the 
source of their disturbance. After they enter the water, they typi- 
cally begin barking, form groups (rafts) , and swim back toward the 
shore with heads held high from the water as though trying to ob- 
serve the cause of their alarm. 

In our experience, the immediate causes of these stampedes were 
human activity, alarm calls or other sudden activity of gulls, escape 
reactions of cormorants or harbor seals, a loud noise such as the 
slap of a flipper on a wet rock, or indeed any sudden movement, 
even that of another sea lion. Sometimes the sight of a bull stand- 
ing at its full height near the inland edge of a rookery causes alarm, 
and on warm and sunny days the stampedes may occur for no ap- 
parent reason. 

This alarm reaction is probably learned by the pups during their 
first few weeks of life. During their first week or so, despite their 
precociousness, they show no fear of man and do not join in stam- 
pedes. But by the time they are about a month old they begin to 
react like the adults. We have already noted that adult Galapagos 
sea lions show virtually no fear of man, so it is entirely possible 
that the alarm reaction is learned and "culturally" transmitted in 
the California population. 







Fig. 8. Close-up of a pup scratching dog-fashion. Note that the toes are flexed 
to expose the claws. 







Fig. 9. Territorial bull charging a man (top) . 

Fig. 10. Bull defending area adjacent to carcass of dead female (bottom) 
Note the long vibrissae and scars on the neck and chest. 



REPRODUCTIVE BEHAVIOR 




CALIFORNIA sea lions are highly po- 
lygynous; the ratio of females to 
males in our study area during the 
1965 breeding season averaged 14 : 1. 
The typical rookery consists of terri- 
torial bulls spaced fairly uniformly in 
a single line along a beach, with fe- 
males in dense aggregations at irreg- 
ular intervals among the bulls. This pattern of spatial organiza- 
tion is determined primarily by the strong gregariousness of the 
females, and by the extreme aggressiveness of bulls. The arrange- 
ment of the territorial bvills depends upon the presence of females 
to a large extent, although in some places the bulls defend terri- 
tories on stretches of shoreline when no females are present. Most 
of the unemployed males gather instead on a few separate beaches, 
and on these hauling grounds they form non-territorial aggregations 
similar to those that exist during fall and winter. 

Breeding Sites.— As is the case in most pinnipeds, breeding in 
Zalophus tends to be restricted to a small number of the potentially 
suitable localities available. Although they tend to breed on the 
same sections of the coast year after year, they tend to be less 
permanently fixed to traditional sites than is the case in Callorhmus 
(Kenyon, 1960:440) . For example, during the breeding season an 
entire breeding aggregation of Zalophus may shift its location by 
hundreds of meters because of repeated disturbance, sustained heavy 
surf, or for no discernible reason. 

On the Channel Islands, Zalophus usually breed on shores ex- 
posed to the prevailing winds, as discussed earlier. Rookeries exist 
on both rocky shores and sandy beaches, but the locally preferred 
habitat appears to be rocky flats and shelves, especially those which 
are occasionally washed by large waves or submerged at high tide. 
Such terrain not only keeps the animals wetted, but is frequently 
cleaned of carcasses and wastes by action of the sea. At present, of 
course, one of the important determinants of rookery location is the 
local pattern of distribution of humans. 



21 



22 The California Sea Lion 

Male Territories 

Shoreline Locations.— Almost all territories of Zalophiis bulls 
extend to the water's edge, a feature that affects the entire system 
of social organization. Access to the sea seems to be required by 
bulls as a part of their habit of keeping continually wetted on sunny 
days. Bulls that station themselves inland of the line of territories 
that abuts on the water are usually transient and often abandon 
their territories after only a few hours. Bulls sometimes establish 
and maintain temporary landlocked territories at night, only to 
abandon them in the morning. Inland bulls often are smaller and 
presumably younger than those in the first row along the shore, 
and thus may not be sufficiently large or experienced to hold a 
stretch of beach in face of competition from other bulls. Occasion- 
ally, tide pools are used for wetting, and a few bulls hold inland 
territories centered on these. 

On San Nicolas Island the vertical tidal excursion is about three 
feet, so that the flat, sandy beaches used for rookeries are awash at 
high tide. Consequently, many male territories are partly aquatic. 
Some bidls even maintain wholly aquatic territories immediately 
offshore from the rookeries, staying on station in the water even 
when the surf is heavy (Fig. 11). 

Size of Territories.— The territories of Zalophus bulls are poorly 
defined. Like those of Arctocephalus (Paulian, 1964:96), they 
vary considerably between individuals, depending upon movements 
of females, temperature, and time of day. The specific territorial 
boundaries are hard to map for several reasons: (1) typically there 
is no inland boundary which a bull defends, (2) the seaward edge 
of the territory may be underwater, (3) a bull only infrequently 
patrols the entire perimeter of his territory, and (4) intruding 
bulls are sometimes tolerated in territories by the resident bull for 
hours at a time, very probably because he remains unaware of them. 
On the spacious, flat beach (Area 6B) east of our observation post, 
many bulls were spaced along the shore at 10- to 15-meter intervals 
during early Jtuie (Fig. 12) ; this probably represents an idealized 
distribution during the breeding season. On the rocky, irregular ter- 
rain that constituted our study area, six or eight territories, recog- 
nizable but irregular in shape, were usually maintained. 



Reproductive Behavior 23 

Duration of Territory Maintenance.— We have seen bulls de- 
fending territories during May, June, July, and August, but the 
territorial behavior is most intense and sustained in late June and 
early July, at which time some bulls even become aggressive toward 
humans. Although territories are maintained over a period of four 
months, individual Zalophus bulls do not stay on land continuously 
throughout the breeding season; in fact, few individuals remain 
at one location for more than two weeks. As discussed in a later 
section, during June and July, 1965, the duration of territory main- 
tenance of 5 different bulls which could be individually identified 
from scars and marks had a mean of 9 days. Several of these 
bulls were seen at other places around San Nicolas Island during 
the summer, but at these times they were not holding territories. 

Two lines of indirect evidence suggest that few bulls hold terri- 
tories much longer than the individuals in our small sample: (1) 
the bulls do not become thin and emaciated toward the end of 
the breeding season as do many fur seal bulls, and (2) Zalophus 
bulls occasionally defecate on land, strongly suggesting that they are 
not fasting for prolonged periods. 

Although the rate of turnover of bulls in the rookery is high in 
comparison with other polygynous pinnipeds that have been stud- 
ied, the spacing and distribution of the bulls remains relatively 
constant throughout June and July. A new bull does not usually 
preempt parts of established territories; instead, it takes over the 
same stretch of shoreline that was held by the animal it displaced. 
As a result, a given territory may be occupied by a succession of 
different bulls during the breeding season. This contrasts with 
CaUorhinus and Mirounga rookeries, which become progressively 
more crowded with bulls during the early breeding season (Peter- 
son, 1965^:82; Laws, 1956:77). 

Establishment of Territorial Status.— We do not know how 
the first bulls in a given season establish their territorial status, 
but once some bulls are established, subsequent individuals must 
fight to gain a place. The motor activities associated with fighting 
appear to be aimed primarily at physically forcing the opponent out 
of a territory. The commonest pattern consists of chest-to-chest 
pushing. Quick, slashing bites and more prolonged grappling ma- 
neuvers accompany pushing, but almost always seem to be directed 
toward the same goal, physical displacement of the opponent. Fights 



24 The California Sea Lion 

occur both on land and in the water. The longest fights take place 
in shallow water, where the animals have greater maneuverability 
than on land, but where it is more difficult for a defeated animal to 
retreat than would be the case in water deep enough for easy swim- 
ming. Fights in deep water are very vigorous but usually brief. The 
defeated individual can readily escape, not only because of the 
agility of sea lions in water, but also because of the three dimen- 
sional nature of an aquatic territory. 

Bulls on established territories bark incessantly. An intruding 
male, in contrast, is silent and rapidly approaches the territorial 
bull that it is going to attack with every indication of purposeful- 
ness but without vocalizing. A fight usually begins as soon as the 
intruder crosses the territorial boundary, since the established bull 
generally moves rapidly to meet the intruder. The ensuing fight 
may last several minutes. It consists of a series of violent struggles, 
separated by brief pauses during which the bulls stand chest-to-chest. 
During these pauses, the contestants hold their heads high in the 
air as if each were trying to tower above the other. Then, as the 
pause ends, each bull weaves its head and neck laterally several 
times and finally lunges again at the opponent's chest, flank, or 
foreflipper. During these lunges the animals often fall flat on their 
chests, necks extended. The massive, well-padded chest and the 
neck receive most of the wounds and seem to act as a shield for the 
more vulnerable flippers. If the animals are half submerged in the 
water, their efforts at seizing each other's flanks and foreflippers 
may lead them into a circular chase, sometimes with each attempt- 
ing to seize the hindflippers of the other. Once a bull is bested, 
either by being pushed out of the territory in question, or by being 
overpowered and apparently losing confidence, he tries to retreat 
while at the same time protecting his vulnerable hindquarters. A 
retreating bull, therefore, repeatedly whirls to face his pursuing 
opponent. 

A fight seldom involves more than two bulls. When the fighting 
pair blunders into the territory of a third bull, this neighbor 
sometimes attempts to bite the flipper of whichever bull he can 
reach. Fights involving several bulls at their territorial intersects, 
as seen in Callorhinus (Bartholomew and Hoel, 1953:484; Peterson, 
19656:67), were never observed in Zalophus. This probably is a 
function of the lower level of aggressiveness, and rather indefinite 



m.. 






Fig. 11. Two bulls maintaining adjacent territories in the water at one edge 
of study area. 




Fig. 12. Typical spatial arrangement in a rookery on a sandy beach with ani- 
mals on damp sand at water's edge. Territorial bulls are spaced at intervals of 
10 to 15 meters, and the confluent female aggregations are arranged without 
reference to the boundaries of male territories. 



Reproductive Behavior 25 

territorial boundaries of Zalophus as compared with CaUorhinxis. 

Maintenance Behavior.— Once a bull is established on territory, 
it does relatively little fighting. A territory rarely borders more 
than two or three others, and established neighbors are not con- 
cerned with displacing each other, but merely with maintaining 
their own territories. Territorial maintenance activities consist pri- 
marily of loud and frequent vocalizations, and of displays with 
formalized postures and movements. The loud and incessant bark- 
ing of a territorial bull continuously advertises his presence and 
broadcasts his intent to defend his territory. When not aimed at 
any specific bull these barks are relatively slow, but if the territorial 
bull is moving toward another male, the rate of barking increases, 
apparently signifying a more intense and directed threat. 

Pairs of established bulls regularly perform a stereotyped boundary 
ceremony, reaffirming their mutual territorial limits. They rush 
toward each other, barking rapidly, with vibrissae extended an- 
teriorly. Just before reaching the boundary between their territories 
they stop barking and fall on their chests, open their mouths widely, 
shake their heads rapidly from side to side, and weave their necks 
laterally at a rate much slower than the head-shaking which is simul- 
taneously going on. They then rear themselves to maximum height, 
twist their heads sideways, and stare obliquely at each other. Three 
stages of this ceremony are shown in Figures 13, 14, and 15. The 
process is ritualized, and the animals do not touch each other; if 
they happen to be unusually close together they skillfully avoid 
contact. Sometimes during the boundary ceremony the two bulls 
partially cross the boundary and stand side by side, facing opposite 
directions, making feinting jabs at the chest and flippers of each 
other. The oblique stare is often the last component of the display 
prior to the bulls' separation, or prior to repetition of the entire 
head-shaking, neck-weaving sequence. 

Temporary Intruders.— The activities associated with territorial 
maintenance by established bulls are not continuous. Periods of 
alertness and movement are interspersed with intervals of sleep, 
often at apparently preferred sites within the territory. During 
these intervals of male inattention, which may last for several hours, 
neighboring territorial bulls show no interest in entering the un- 
tended territory. Non-territorial, wandering bulls, however, often 
move cautiously into a territory at such times, usually "hiding" 



26 The California Sea Lion 

among the resident females. These intruders may be fully grown, 
or they may be small animals the same size as the females. They 
usually approach from the landward side, having landed at some 
undefended place and wandered inland to the rookery, although 
sometimes they enter from the sea. They do not vocalize, nor do 
they challenge or otherwise disturb the bulls in adjacent territories. 
Often they show sexual interest in the females, and may attempt to 
mount them. When one of these intruders is discovered by the resi- 
dent bull, a vocal threat is ordinarily enough to send him rushing 
away. Sometimes the intruder remains undiscovered for hours, and 
we observed one that remained quietly (apparently unnoticed) in a 
territory for three days. Intruders seem to remain undetected as 
long as they are visvially inconspicuous and do not respond with 
threatening or rapid movements to the activity of the resident bull. 
Thus, there is a premium on their behaving very much like females. 
Perhaps because of poor aerial vision bulls do not readily recognize 
these intruders as males. 

Early in the breeding season, most of the temporary intruders 
are fully grown bulls, evidently from the hauling grounds. Toward 
the end of the season, they are more often young males about the 
size of the females. A corresponding decrease in the number of 
adult males on the hauling-ground population occurs as the season 
progresses. If fully grown and allowed to remain for many hours 
without being challenged by the resident bull, a temporary in- 
truder may gain enough confidence to stand its ground briefly and 
return the threat of the resident. But although they sometimes 
fought with the resident, we never saw the temporary intruders 
successfully establish themselves as territorial bulls. 

The frequency of these intrusions is surprisingly high, evidently 
because the linear arrangement of the territories makes almost every 
territory accessible to wandering animals from both the landward 
and the seaward sides. During the period between 1100-1400 hours, 
14 June 1965, we saw four temporary intruders enter seven terri- 
tories at the study area. Later in the season, small males sometimes 
dashed into the aggregations of females every few minutes, fre- 
quently unnoticed by the resident bulls, and often unnoticed by us 
until they bolted from among the females when the resident 
threatened them. 



Reproductive Behavior 27 

Because these temporary intruders are transient, and almost never 
defend their positions, we do not consider the territories to be 
composite, nor the intruders to be "subordinate breeding males" 
as in Mirounga (Bartholomew, 1952:394; Carrick et al, 1962:173). 

Relations of Female Aggregations to Male Territories.— The 
individual females on a rookery pay no attention to the territorial 
boundaries maintained by the bulls. The bulls patrol the bound- 
aries irregularly and make no effort to herd females into their 
territories or to hold them there. Consequently the arrangement 
of the female aggregations shows no regvilar relationship to the 
territories of the males, nor is a given female confined by a bull to 
any one territory. Therefore, the word "harem," so firmly estab- 
lished in the literature on pinniped behavior, is not applicable to 
Z. c. calijornianus. Eibl-Eibesfeldt (1955) uses the German term 
Herd, suggesting harem, for the Galapagos sea lion. 

Although the females are relatively independent of bulls' terri- 
tories, the converse does not hold. Stable and organized territories 
exist only when and where females are present, and shifts in the 
females' locations modify the territorial activities of the bulls. Such 
shifts in the locations of the females occur erratically, with storms or 
disturbances, and regularly, with the tidal and day-night cycles. 
The tidal cycle in California has a duration of approximately 12 
hours; the females generally stay near the water's edge, shifting 
back and forth on the beach with the tide. The territorial bulls 
generally move back and forth with them; the lateral and seaward 
boundaries of the territories are fixed but the inland boundaries are 
less definite. At night, the females usually move back to the inland 
edge of the beach and sometimes even go above the beach onto 
vegetation-covered terraces. The territorial bulls tend to accompany 
the female groups as they move inland at night, thovigh they still 
continue to defend the seaward edges of their territories. If the 
females abandon a stretch of shore completely, as they sometimes 
do in the face of a heavy surf, the bulls remain on the vacant beach 
for only a day or so before deserting their territories. Occasionally, 
however, one finds a stretch of shore on which bulls are widely 
spaced and aggressive toward each other even though females have 
not been ashore there. We do not know how long bulls maintain 
their stations in such areas, or why they are aggressive there, but 
not aggressive on the hauling grounds. 



28 



The Califoryiia Sea Lion 



Nocturnal Activity.— W^e frequently visited the rookeries and 
hauling grounds at night, and on 30 June 1965 we maintained an 
all-night watch at the study area. In the rookeries, activity was, if 
anything, greater at night than during the day. Although some of 
the females and many of the pups slept, sleeping seemed no more 
common at night than during the day. All of the usual activities 
seen during the day also took place at night. Vocalizing continued 
unabated. Some females went to sea; others returned to suckle their 
pups. Pups played, wandered about, slept, or suckled. The terri- 
torial bulls were somewhat more active at night than during the 
clay. They moved about more continuously and appeared more 
zealous in their inspection of females and their chases of intruding 
males. It seems probable that the heightened level of activity of the 
bulls at night is associated with the fact that over-heating is not as 
acute a problem at night as it is during the day. 

In contrast to the situation on the rookeries, the level of activity 
on the hauling grounds frequented by the non-territorial males was 
much lower at night than during the day. Most of the animals on 
the hauling grounds moved inland and slept. 

Behavior of Females 

Gregariousness.— The polygynous so- 
cial order of this species results from 
two basic elements, the male territory 
and the female aggregation, both of 
which vary seasonally as do the relative 
aggressiveness and the relative gregari- 
ousness of male and female. Arriving 
pregnant females are clearly attracted 
to the vicinity of other females already on land. Although just 
after parturition the heightened aggressiveness between females 
produces a quasi-territorial pattern among them, one rarely sees 
a female-pup pair isolated by itself. Almost always the females re- 
main in aggregations, albeit at the height of pupping the aggrega- 
tions become rather loose. Later in the season the aggressiveness 
between females diminishes markedly and the females and their 
pups are nearly always found concentrated in aggregations. It 
should be emphasized that throughout all of the female's activity, 
some degree of gregariousness persists. 





Fig. 13. Figures 13, 14, and 15 show typical patterns of movement in the 
boundary ceremony of adjacent territorial bulls. In this photograph the animal 
on the left has just lunged forward and both animals are shaking their heads 
from side to side. 




Fig. 14. Boundary ceremony continued; the oblique stare 



Reproductive Behavior 29 

Arrival at the Rookery.— Pregnant females apparently land at 
the rookery only a day or so before giving birth. They are noticeably 
bulky, walk with relative difficulty, and are restless and irritable. 
Sea lions are conspicuously vocal, but the pregnant females are very 
quiet and rarely if ever vocalize. 

The bulls generally pay little attention to the individual females as 
they accumulate on the rookery, but sometimes they bark at them 
and approach and sniff at their noses. Infrequently, a bull will 
attempt to nuzzle a newly arrived female's flanks or rump. The 
females generally ignore or avoid the bulls, but sometimes they 
react to the approach of a bull with open-mouthed threats or by 
retreating. Only infrequently do bulls attempt to "herd" females 
into their territories by threatening or blocking their way. When 
it does occur, such behavior is much less vigorous than that of 
Callorhinus bulls (Bartholomew, 1953) . We never saw Zalophus 
bulls make either prolonged or effective efforts to retain females in 
their territories. Usually a territorial bull gives no response to 
arriving or departing females, even if in full view of the bull. In 
this species we saw nothing which we could interpret to be a 
"harem." 

Aggressiveness Between Females.— Even though they are gre- 
garious and almost always gather in groups, a conspicuous feature of 
the behavior of Zalophus females during the the breeding season is 
their aggressiveness. As mentioned above this aggressive behavior 
is heightened considerably at the time of birth when parturient fe- 
males are virtually territorial {cf. Eibl-Eibesfeldt, 1955:293) . 

Most of the aggressiveness of the females is quite formalized and 
despite its vigor and noisiness injuries are rarely inflicted. Typi- 
cally, an aggressive female shifts about restlessly near her pup, 
barking continually and rushing toward any neighbor that ap- 
proaches too closely. She threatens by extending her neck and 
twisting her head to one side with mouth open wide. The female 
being threatened responds in a similar manner. Thus, one often 
sees pairs of females weaving their heads and necks back and forth 
and vocalizing (Fig. 16) . Sometimes the two animals freeze in 
this head-aside posture for several seconds. If the contest is not 
terminated at this stage, they may begin biting each other on the 
necks or flanks and sometimes they roll and writhe on the ground 
like wrestlers, but inflict no wounds. Finally, one of the individ- 



30 The CaUjornia Sea Lion 

uals withdraws a few feet and the squabble dies away. These inter- 
actions are very common and, since they chsturb adjacent females, 
tend to be contagious. Sometimes three or four, or even more, fe- 
males become involved. But the aggressive displays are brief 
(a minute or less) and usually result in nothing more than minor 
shifts of location. If the aggregation is startled, as by the alarm call 
of gulls, the resulting changes in position of the females stimulate 
a great many aggressive interactions. 

We have seen no evidence of stable dominance hierarchies, as 
occur in Mirounga females (Bartholomew and Collias, 1962:8). 
The absence of such hierarchies may be related to the greater mo- 
bility of Zalophus and the resultant decrease in numbers of con- 
tacts between the same individuals; Mirounga females lie almost 
immobile near each other for several days. Transient dominance- 
subordinance relations between pairs of Zalophus females do occur: 
pregnant females, females without pups, or females en route to the 
sea usually behave subordinately. 

Aggressive Vocalizations.— There are at least four vocalizations 
associated with the inter-female aggressive behavior. The com- 
monest is the hark which is similar to that of the bull but higher 
pitched. It identifies and localizes a female and serves as a relatively 
mild, long-distance threat. When two contesting females are close 
together, three intergrading vocalizations are used. The least in- 
tense of these is a squeal which is of variable pitch, sometimes quite 
high. It is made with wide-open mouth and usually accompanies 
the head-weaving behavior sequences. The belch is a somewhat 
more intense threat, a prolonged, harsh, gagging sound, usually 
accompanied by forward thrusting of the head. The most intense 
vocal threat is an irregular growl which has a harsh, erratic, aspirate 
quality and is often vised during actual strikes with the head or 
during biting. 

"Peace-keeping" by Bulls.— The noisy squabbles of females some- 
times evoke a response from the nearest territorial bull which begins 
to bark loudly, moves rapidly toward the females involved, and may 
attempt to sniff or nuzzle them. Eibl-Eibesfeldt (1955:292) inter- 
preted this behavior in the Galapagos sea lion to mean that "Das 
Mannchen duldet keinen Streit unter den Herdenmitgleidern" and 
sviggested further that bulls actively prevent females from fighting. 



Reproductive Behavior 31 

Steller (1751; translation: 203) ascribed similar meaning to the re- 
actions ol; fur seal bulls to squabbles in their territories. 

We saw nothing in our study to which we would apply such 
interpretations. Bulls react in a similar "peace-keeping" manner to 
any disturbances within their territories. For example, to young in- 
truding males or especially noisy pups at play. It is reasonable to 
assume that this response of bulls is simply a general reaction to 
heightened activity in their territories. The bull's approach may 
temporarily halt the fighting between females, by frightening them 
into fleeing a short distance, but it certainly does not pacify them. 

Parturition 

Our daily censuses indicate that most of the pups were born dur- 
ing June. Pregnant female sea lions are easily recognized by their 
distended abdomens and laborious locomotion. Although we 
watched them carefully throughout the pupping season we wit- 
nessed only two deliveries, one at 1715 hours on 20 June, and one 
at 1900 hours on 22 June 1965. Since new pups were present on the 
rookery almost every morning during the pupping season, we con- 
clude that most deliveries occurred at night. 

During parturition, the females became increasingly restless as 
they passed through the first stage (vaginal dilation) and fre- 
quently turned to nuzzle the perineal region, sometimes circling 
about like a dog chasing its tail. During this stage both the females 
we observed uttered a few, brief pup-attraction calls (see section on 
mother-young recognition) , but otherwise were silent. This vocali- 
zation was directed toward the perineum, as if in anticipation of 
delivery. 

Both deliveries were by anterior (cephalic) presentation (cf. 
Slijper, 1956:42) . In one birth which was timed, the second stage 
lasted 2.5 minutes. More than two minutes were reqvured for the 
pup's head and shovilders to appear, and then movement was rapid 
as the thorax, the thickest part of the spindle-shaped fetus, was 
delivered. Thereafter, the hindflippers appeared almost immedi- 
ately. The umbilical cord was broken, in both cases, by the time 
the pups were delivered. The third stage (placental delivery) oc- 
curred after 17 minutes in one case, and did not take place for at 
least one hour in the other case. 



32 The California Sea Lion 

The new pvips lay on the rocks quivering for a few seconds, then 
began to struggle weakly, made feeble attempts to stand on their 
foreflippers, and succeeded in doing so within a minute or so. 
They shook themselves like dogs several times dviring the first few 
minutes, jtist as wet adults do, and soon began vocalizing. 

Courtship and Copulation 

Females come into estrus about two weeks after parturition. 
The vulva of an estrous female becomes pink and slightly edema- 
tous, and is therefore much more conspicuous than in other females. 

Courtship.— Territorial bulls are relatively inactive in courtship. 
They do not examine the females that happen to be in their terri- 
tories with any regularity or system. However, bulls often attempt 
to sniff the noses of females that are moving about, and will nuzzle 
persistently at the genital area of any female that is lying prone 
and is at all receptive. Sometimes a bvdl will move about a female, 
barking and shaking his head, much as in the formal aggressive 
encounters between males. ^Vhen used in courtship, however, this 
behavior assumes a rather tentative quality and may culminate in 
the bull's rubbing his whiskers along the female's body and nipping 
at her shoulders and sides (Fig. 17) . Such a courtship performance 
by the bull is by no means invariable and is always less vigorous 
than in Callorhiniis. 

When a Zalophus female comes into estrtis she usually solicits the 
male to mount her. This contrasts sharply with other pinnipeds 
in which the male plays the active role. The Zalophus female ap- 
proaches the bull, and by characteristic submissive posttires and 
languorous movements (estrous display) invites sexual attention. 
An estrous female lies prone in front of the bull, on her side, belly, 
or back, and repeatedly presses her body against his. She executes 
slow, writhing movements on the ground, rubbing against the 
bull, stretching, and looking up at him. Sometimes while making 
these twisting movements she slithers across the back and shoulders 
of the male, almost as though moimting him. Usually, but not 
invariably, the male soon begins sniffing at her genital region. She 
responds by arching her back upwards and spreading her hind- 
flippers. The male generally mounts after a few minutes, though 
sometimes courtship continues for an hour or longer. 




Fig. 15. Boundary ceremony continued; elevated posture with head shaking 




Fig. 16. Open-mouthed threats by females 



Reproductive Behavior- 33 

Copulation on Land.— In copulation the female lies prone, 
usually on her belly, and the bull mounts her from the rear. The 
front limbs of the male are short and the female is relatively large, 
so even when the bidl supports himself on his foreflippers, his chest 
and belly rest on the female (Fig. 18) . Occasionally a btdl rests his 
entire weight on the female and clasps her with his foreflippers. 
The bull's pelvic thrusts are intermittent. He usually mounts and 
dismounts the female several times before copulation is terminated. 
Between mountings the female resumes estrous display. The male 
may cover her when she is lying on her side or even on her back and 
make pelvic movements. These copulatory activities, including 
mountings during which the pair rub their necks together, may con- 
tinue for an hour or more and the pair may even fall asleep for a 
few minutes while lying on or against each other. We were unable 
to tell whether or not intromission occurred during all the mount- 
ings. In all instances observed, copulation was actively terminated 
by the female; she raised her head and forequarters, bit at the neck 
of the male and pulled free, even though the bull was often still 
making pelvic movements. 

Once a copulation has been terminated by the female, her behav- 
ior changes abruptly. She rebuffs the approach of her former 
partner or other sexually interested males with jerky, spasmodic 
movements of the head, not seen at other times, and also actively 
retreats from them. We never saw a given female carry out more 
than one series of estrous displays, nor participate in more than 
one prolonged copulatory series, nor remain sexually active after a 
completed copulation. However, if the first male solicited by an 
estrous female is lethargic or unresponsive, she sometimes moves to 
an adjacent territory and directs her displays at the male there. 
In one case a male repeatedly mounted a female over a period of 
about IV2 hours, but for some reason his activities were inadeqtiate. 
The female then moved to another bull in an adjacent territory 
and courtship and copulation continued for an additional 15 min- 
utes before she became unreceptive. 

For the most part sea lions show no response to or apparent inter- 
est in the behavior of a sexually active pair. Once (13 July 1965), 
however, we observed a female swim up to a pair which were cop- 
ulating in the water, sniff at the head of the male, and then nip 
him gently first on the flank and then on the neck. He made no re- 



34 The Calijornia Sea Lion 

sponse other than to sniff at her nose when she approached. On 
another occasion (4 July 1965) , two females on land were simul- 
taneously courting the same territorial bull. During a period of 
about half an hour he repeatedly mounted each female, sometimes 
lying across both of them at the same time. Finally, he completed 
a copulation with one of the females which then bit him on the 
neck, pulled free and moved rapidly away. The male followed her 
for about 10 meters, but she rebuffed him with the spasmodic head 
movements referred to above. After about three minutes the male 
returned to the second female who promptly resumed her sexual 
displaying. The male mounted her and copulation proceeded. 
After 5 minutes the second female bit the neck of the male and 
pulled free and moved away. The male did not follow her. 

During a prolonged courtship, a bvdl may leave the female and 
exchange vocal threats with another bull, but he becomes relatively 
unresponsive to his neighbors while he is sexually engaged. Sexual 
behavior takes precedence over aggressive behavior; we never saw 
a bull interrupt copulation to engage in aggressive interactions with 
another bull. 

Copulation in the Water.— Since many bulls' territories are 
partly aquatic, and since females usually stay near the water's edge, 
copulation often begins when the two animals are partly in the 
water. If in shallow water, copulatory behavior follows the same 
general sequence as on land, except that the female rests on the 
bottom under water and must occasionally lift her nose to the 
surface to breathe. 

We also observed a number of copulations in deep water, where 
neither the male nor the female was in contact with the bottom. 
Courtship, usvially brief, is similar to that on land except that the 
animals are swimming. To mount, the bull clasps the female with 
his foreflippers, holding her against him. The pair sometimes roll 
over in the water, float on their sides, or sink entirely under water 
and wash back and forth in the surge, breathing only intermittently. 
Judging from its frequent occurrence and duration, aquatic copu- 
lation appears to be successful in Zalophus, a point that has been 
much argued for CaUorhinus (Bering Sea Tribunal of Arbitration, 
1895) . 




MO THER - TO UMG RELA TIONS AND 
ONTOGENY OF PUP BEHAVIOR 

Perinatal Behavior 

PINNIPEDS are among the most precocial 
of mammals (Nice, 1962:17) and Zo/o- 
phus conforms to this generalization. The 
pups have their eyes open at birth and within 
10 to 15 minutes after delivery they can carry 
out highly coordinated motor activities. For 
example, a newborn pup can shake itself like 
a wet dog, scratch with the hindflippers, rvib 
its flanks with the foreflippers, and use its 
incisors to nibble and groom the pelage. Walking becomes coor- 
dinated within the first 30 minutes. Vocal interchanges with the 
mother begin at birth, and the pup almost immediately develops 
the capacity for effective vocal communication. 

Post Partum Mother-young Interactions.— Shortly after delivery, 
the female begins nuzzling her newborn pup, rubbing her vibrissae 
over it and (perhaps incidentally) brvishing away bits of fetal mem- 
brane that may be sticking to its pelage. She repeatedly lifts the pup 
to her side with her teeth and nips at its body. Although this 
nipping and nuzzling continued for at least 10 minutes, we never 
observed a female lick her pup {contra Eibl-Eibesfeldt, 1955:295) , 
nor eat the fetal membranes or placenta. 

Immediately after birth the pup and its mother begin an initial 
series of vocal interchanges, which continue for 15 to 20 minutes 
with little pause. The females make a bawling, trumpet-like pup- 
attraction call and the pups respond with the mother-response call, 
a quavering vocalization reminiscent of the bleat of a lamb. As in- 
dicated previously, in the two births which we observed the females 
began to utter the pup-attraction call shortly before the pup was 
delivered. During the first several hours, these vocal interchanges 
between pup and female are repeated a number of times. In the 
intervals between these vocal interchanges the pair spends much of 
the time sleeping. This post partum vocal interchange anticipates 

35 



36 The California Sea Lion 

the pattern of vocalizing that takes place between a female and her 
pup when they are re-uniting to suckle after separation. Thus, it 
may help establish the mutual recognition of mother and young 
(see Collias, 1956, for a discussion of this problem in sheep) . 

We were surprised to observe that parturient females are nearly 
as attentive to their neighbors as to their newborn pups. They bark 
vigorously and make frequent, threatening dashes at adjacent ani- 
mals. We saw one female leave her pup as soon as it was delivered 
and rush over and threaten a neighbor, before returning to nuzzle 
her pup for the first time. This aggressive, almost territorial, behav- 
ior seems to have priority over all other activities at this stage, even 
over preliminary interactions with the pup. 

Mother-young, During First Few Days.— Females are very pro- 
tective of their pups for the first two to four days, and may even 
refuse to leave them if approached by a man. Physical contact be- 
tween mother and young is maintained almost continuously, the 
female repeatedly tugs or lifts her pup to her side by the loose skin 
of its nape and shoulder region whenever she shifts position slightly 
or the pup starts to wander away. If sufficiently disturbed, by 
weather or human activity, a female with a new pup will carry or 
drag it in her mouth to a new location. Sometimes this involves 
carrying the pup to sea. On 7 June 1965 we watched a female drag 
her pup by its nape, first to the shore and then out into the surf 
for perhaps 50 meters. The pup struggled frantically whenever 
released from the female's mouth, and several times it tried to crawl 
onto her shoulders in a manner similar to that described by Kenyon 
(1956) . The female repeatedly caught the pup and lifted it above 
the surface. Twelve minutes after entering the water, they drifted to 
shore about 400 meters distant from where they started. The female 
helped the pup through the surge and onto the beach, and finally 
the pair joined a group of females and pups in the new area. 

Similar shifts in location occur on land. When a pup is less 
than a week old, the female may carry or drag it; otherwise it follows 
along behind her, vocalizing repeatedly. In a few unusual cases, 
we observed females continuing to carry their pups cat-fashion 
when 2-3 weeks of age. If the move involves climbing over rocks 
or ascending ledges, a female tugs roughly at her pup and may 
drop it several times. Partly as a result of this treatment, many 
young pups have scratches and lacerations on their napes, although 






Fig. 17. Neck-rubbing prior to copulation (top) . Note conspicuous size dif- 
ference between male and female. 

Fig. 18. Typical copulatory posture (bottom) . This picture was taken shortly 
after Fig. 17. 





Fig. 19. Pups playing in shallow water near shore 



Mother-young Relations 37 

the scratches may also resuk from forceful rebuffing by other fe- 
males. As pointed out previously, the female aggregations gradually 
shift position with the tide and daylight, sometimes moving 30-50 
meters on a flat, open beach. During these slow changes in location 
the pups usually follow their mothers without being carried. 

On sunny days when overheating is a problem, females make re- 
peated trips to tide pools or the edge of the sea, sometimes every 
hour or so, and get themselves wet. If pups are less than a week old, 
the females take them along and drag them into the water. The 
pups flounder to shore and try to crawl out on land, but the females, 
apparently reluctant to be separated from their pups, pull them 
back into the water. This pattern, which appears to involve a 
combination of behavioral thermoregulation and mother-young ties, 
is probably the basis for the familiar folk belief that sea lions teach 
their pups to swim. On first sight, it does appear quite like a "swim- 
ming lesson." 

Sometime during the first day, the pup starts to suckle. As 
pointed out by Eibl-Eibesfeldt (1955:296), there is no stereotyped 
teat-searching behavior in Zalophus pups. Newborns nuzzle about, 
apparently at random, until they find the site of one of the retracted 
nipples, although later they learn to locate the site rapidly. 

After approximately four days, the females no longer remain 
continually with their pups. At first, separations may be very brief, 
but as the season progresses, the females spend more time at sea, 
and the pups begin to wander about the rookery, depending on 
their mothers only for nutrition and not for protection. 

Pup Behavior 

The terrestrial locomotion 
of pups during their first 
months is much less variable 
than that of the adults, con- 
sisting of only two patterns: a 
slow walk, and a faster gallop. 
Both patterns are employed 
shortly after birth, but pups become noticeably more agile as the 
weeks progress. 

Pups apparently can swim at birth, but their first efforts are 
awkward, consisting of rapid rotary movements of the forelimbs 




38 The California Sea Lion 

while the head is held high above the water. This pattern is 
strikingly similar to the terrestrial gallop, which also employs simul- 
taneous rotary movements of the foreflippers. Swimming skill de- 
velops rapidly. Breathing coordination and use of the flippers im- 
proves steadily and by mid-July many of the pups spend long 
periods in the water (Fig. 19) . 

Pups spend much of their time sleeping. Their sleeping postures 
(Fig. 6) are similar to those of adults. If the terrain is flat the 
sleeping pups orient themselves randomly, but on a slope they tend 
to lie with heads uphill. 

By mid-June, when many of them are two to three weeks old and 
their mothers are no longer in continuous attendance, the pups 
begin to aggregate in groups. Thereafter, these pods of pups become 
a progressively more conspicuous feature of the rookery. Pups are 
very gregarious: if one becomes isolated while sleeping, its first 
act upon awakening is to rush frantically to the nearest pod. The 
pup pods may include as few as 5 or 6 or as many as 200 individuals. 
The composition of a given pod is always in a state of flux; individ- 
uals and small groups frequently join it or move away, and oc- 
casionally the entire group moves off to a new location. 

Typically, these pods are located near the female aggregations, 
and shift with them diurnally and tidally. Pups aggregate in areas 
where the territorial activity of the bulls is minimal. Thus, they 
accumulate on the edges of the rookeries, among large boulders, 
or beneath overhanging ledges. Bulls pay little attention to pups, 
although sometimes it appeared that a bull would slow somewhat 
before rushing through a pod, permitting the young to scramble 
out of his way. We saw very few pups run over by bulls. 

Pups appear to be less sensitive to heat and more sensitive to cold 
than adults. They show no tendency to keep continually wetted 
on warm days, and unlike adults, when they are wet, they often 
shiver and repeatedly shake water from their pelage. From these 
responses we infer that the young animals are poorly insulated and 
dissipate heat readily, as is the case with Callorhiniis pups (Barthol- 
omew and Wilke, 1956:331; Irving et al., 1962:276). 

Pup Vocalization.— The only vocalization produced by newborn 
pups is the bleating mother-response call, but after they become 
older and form pods, they begin to use three other intergrading 
vocalizations. If a pup is struggling, as during play, it occasionally 



Mother-young Relations 39 

emits an explosive, unvoiced cough, or a rapid series of them which 
sometimes become voiced. If a pup becomes agitated, it often 
prolongs the voiced portion of the series, and produces a relatively 
high-pitched alarm call. In early August, when many pups are 
about six weeks old, they begin to develop the adult-like bark, a 
threat which seems to arise as a segmented version of the alarm call. 

All three of these vocal patterns have social significance, evoking 
various responses from nearby pups and adults. We shall discuss 
vocalization further in a separate paper. 

Play of Pups.— The play of sea lion pups resembles that reported 
for northern fur seals (Bartholomew, 1959) . From the time a pup 
first joins a pod it engages frequently in elaborate play behavior 
which appears to have no immediate relevance to nutrition, repro- 
duction, or protection, but may adumbrate adult behavior. 

Individual pups exhibit many playful motor patterns: rocking 
laterally from one foreflipper to the other, rolling onto the back 
and waving the flippers in the air, and elevating the forequarters 
while simultaneously bending the neck back sharply and peering 
about over the back. 

Most of the pups' play is social, however. Sometimes two or three 
pups rub their heads, necks, and bodies together, rotate their 
vibrissae forward, and touch noses. They often nip each other's 
faces and forequarters, or gently interlock jaws. Some of these ac- 
tivities assume the stereotyped configurations of adult aggressive 
interactions. For example, open-mouthed threats, feinting lunges at 
flippers and necks from a prone posture (Fig. 20) , and mounting 
with sketchy copulatory movements are frequently seen. 

Sometimes large groups of pups engage in contagious patterns of 
play. A pod may travel <?n masse through the rookery, moving er- 
ratically and without regard to the bulls' territories. One or two 
pups often appear to lead these mass movements temporarily, while 
the others gallop along behind, frequently stopping to nip each 
other or roll on the ground in a brief scuffle. These wandering 
groups seem to be attracted to conspicuous objects in the rookery 
such as dead animals or tide pools. 

By the time pups are about six weeks old they make short explora- 
tory trips inland from the rookery singly and in small groups, sniff- 
ing and rubbing against boulders, plants, or concealed human ob- 
servers. As the season progresses they wander more and more widely, 



40 The California Sea Lion 

venturing along the coast and out to sea for short distances. By 
late July the tide pools and sheltered inlets in the vicinity of the 
rookery are often filled with bobbing pups, engaged in protracted 
and elaborate aquatic play (Fig. 19) . They submerge their heads, 
peer about under water and chase each other over partially sub- 
merged cobbles and boulders, frequently touching or nipping each 
other. We watched one pup that seemed quite entertained by im- 
mersing its muzzle repeatedly and blowing streams of bubbles from 
its nostrils. 

Pups also play with inanimate objects. They are strongly at- 
tracted to pieces of the giant kelp (Alacrocystis) which commonly 
wash ashore, and they endlessly shake and toss them about. Several 
pups may successively play with the same piece of kelp, and occa- 
sionally two may tug on a piece at the same time. Small pebbles 
and sticks are also picked vip and held in the mouth, although we 
never saw them thrown, as Eibl-Eibesfeldt (1955:297) describes for 
Z. c. ivollebaeki. Some pups energetically gnaw at rocks or banks, 
occasionally packing their mouths full of sticky sand. 

Mother—Young Recognition and Stickling 

Suckling.— A pup soon learns the location of its mother's four 
retractable teats and switches from one to another as it suckles 
(Fig. 21) . The nipples emerge during suckling; whether extruded 
actively or passively we could not determine; within a few seconds 
after a pup releases a nipple, it retracts. The process of suckling is 
surprisingly noisy, and smacking noises can easily be heard from a 
distance of 10 meters. As Eibl-Eibesfeldt (1955:296) has described 
in Z. c. ivollebaeki, the pups use suction to obtain milk, rather than 
massaging the teat with the tongue as some mammals do. Except 
for lying on her side and exposing the area of the teats, the female 
does not appear to assist the suckling pup; frequently she sleeps 
while the pup feeds. 

The usual pattern is for a pup to suckle steadily, with short 
breaks, for about one-half hour, and then separate from its mother. 
The mean, minimum, and maximum of five periods of suckling, 
timed during daylight hours on 17 July 1965, were 33, 20, and 48 
minutes. After such a period, the pup appears to be satiated and 
wanders away from its mother. We seldom saw a pup which was 



Mother-young Relations 41 

more than two or three weeks old engage in more than one such 
suckHng period in a single day, but very young pups may suckle 
intermittently for several hours. As discussed in detail later, pups 
reunite with their mothers and feed at irregular intervals, seldom 
more often than once each day or less frequently than once a week. 

Maintenance of Mother-pup Ties.— We were able to mark four 
female-pup pairs shortly after parturition (judged by fresh placen- 
tas or by witnessing birth) , and we saw these pairs reunite repeatedly 
during the three months following. We never observed among these 
eight individuals any suckling which did not involve a mother and 
her own pup. We also kept records of a number of other female- 
pup pairs which although not marked immediately after parturi- 
tion were assumed to be mother and young because of the female's 
attentiveness and the fact that she suckled the pup. M^'e never saw 
a female suckle any but her own pup nor did we see any female 
accept two pups, either simultaneously or successively. Therefore, 
we conclude that a Zalophus female recognizes her own individual 
pup and nurses it to the exclusion of all others. Similar integrity 
of the mother-young bond has been described for Callorhinus 
(Bartholomew and Hoel, 1953:420-21) and Mirounga (Bartholo- 
mew, 1952:391), although fosterage frequently occurs in Halichoe- 
rus (E. A. Smith, 1965:73-74). 

Mother-young recognition probably becomes mutual by the time 
a pup is about two months old, but prior to this, the integrity of the 
pairs appears to depend on the female. Young pups respond posi- 
tively to any searching female, but the female rebuffs all pups but her 
own. The usual rebuff is an open-mouth threat but sometimes the 
female seizes the pup in her teeth and tosses it randomly into the 
air. This maneuver is usually performed awkwardly and sometimes 
the pup is thrown against the female instead of away from her. The 
rejection is obvious, however, and the pup always retreats rapidly. 

Mother-young Recognition.— When a female comes ashore and 
starts searching for her pup, she vocalizes repeatedly, approaches 
any pups which respond, and appears to examine them both olfac- 
torily and visually. When she finds the proper pup the process of 
mutual identification generally takes only a minute or two, and 
suckling begins shortly thereafter. Since the reuniting of mother 
and pup usually takes place before the female has been ashore long 
enough for her pelage to dry out (5-15 minutes) , we infer that 



42 The California Sea Lion 

this behavior has a high priority when a female returns from feed- 
ing at sea. 

A rookery may contain hundreds of females and pups. The 
probable clues by which females and pups might identify each 
other individually are few. Because we could not perform experi- 
ments without disturbing the group we were studying, we have 
attempted to assess the relative importance of several probable 
recognition factors by repeated observations of the process under 
undisturbed conditions. 

1) Geographic location: Neither females nor pups appear to be 
tied to a particular geographical site. Newborn pups may be moved 
from one place to another by their mothers; older pups move with 
their mothers for long distances, and the reunion and suckling of a 
given pair occur at different sites at different times. From these 
facts we conclude that strict geographic localization within the 
rookery is not a primary factor in bringing a Zalophus female and 
pup together, and that there is no one place to which a given pair 
returns for reunion. Geographic localization could, however, offer 
an important preliminary clue if, after each trip to sea, the female 
returned to the approximate site at which she last suckled her pup. 
Such a pattern would allow for shifts in location if the shifts oc- 
curred when mother and pup were together. Our data suggest that 
a pup, if it becomes separated from its mother for a prolonged 
period, tends to remain at, or return to, the place where it was last 
suckled. Pup Number 4, for example, was apparently unattended 
by its mother from 14 to 20 June. During the period, it spent much 
of its time near one site, where eventually its mother found it. How- 
ever, the pair was never observed suckling at this location again. 

2) Vocalization: We assume that mother and pup learn to recog- 
nize each other's voices during the repeated vocal interchanges 
which begin immediately after birth and continue for several days 
thereafter. On scores of occasions we have watched females land at 
the rookery, stand with head high, trumpet the penetrating pup- 
attraction call and then move toward a nearby pod of pups. Usually 
a female will repeat the call several times per minute at irregular 
intervals. Typically, one of the pups in the vicinity responds to her 
call and moves directly toward her. Sometimes the pup is silent, 
but it usually replies with the bleat-like mother-response call, and 
then female and pup continue to vocalize alternately while moving 



Mother-young Relations 43 

toward each other. Several pups sometimes approach a vocalizing 
female, but usually one is more vigorously responsive than the 
others. At night when we had crawled into a pod of sleeping pups, 
we several times saw individual pups suddenly rouse, reply to a 
distant pup-attraction call by uttering the mother-response call, 
and then move away to join the vocalizing female while the other 
pups in the pod remained sound asleep. The selectivity of response 
of both mothers and pups was impressive, particularly in August 
and September by which time the pups were 2-3 months old. Our 
observations left us convinced that vocal discrimination plays an 
important role in mother-young recognition. 

3) Vision: Pups frequently respond positively to the sight of a 
female if holding her head high, particularly if she is walking out 
of the water onto the shore, and they may attempt to approach and 
nuzzle her even if no vocalization has occurred. This response is of 
low intensity and may occur simultaneously in several pups. Visual 
clues, although obviously used to orient female and pup toward 
each other, do not seem to contribute to specific individual identi- 
fication. This is not surprising in view of the limited visual acuity 
of the species and in view of the fact that identification takes place 
at night as well as during the day. 

4) Oljaction: From the frequency and persistence with which a 
female nuzzles and sniffs at her pup, we infer that olfactory infor- 
mation is important in close-range identification, as it seems to be 
in goats (Klopfer et ah, 1964) . Once a female and pup have come 
together, they invariably touch noses and sniff at each other; this 
examination appears to complete the process of recognition. With- 
out experimental evidence, we cannot say categorically that olfac- 
tion is the essential element in mutual recognition, but from field 
observation the case is convincing, particularly since the process 
frequently proceeds to the nuzzling stage before pups are rejected 
or accepted. We conclude that a female confirms the identification 
of her pup by its individual odor. 

5) Other clues: Other possible sources of information that could 
be used for recognition are taste and touch. We cannot exclude 
these, since the female and pup rub whiskers and may touch each 
other's noses, but these sensory modalities do not appear to be of 
major importance. 



44 The California Sea Lion 

Seasonal Changes in Mother-young Relations.— As the summer 
progresses, pups and females spend progressively less time at the 
rookery where parturition occurred, and they seem to spend less 
time together on land. Since we often saw them depart together, 
and swim along near each other, it appears that increasing periods 
of time are spent together at sea during autumn months. 

The mother-young tie may be maintained a year or longer. On 
4 February 1950 on Islas San Benitos, Baja California, eight yearling- 
sized individuals were seen suckling, as were four others at San 
Nicolas Island on 23 April 1950, at a time when no pups had yet 
been born. During June and July 1965 we repeatedly observed 
suckling by individuals which could hardly have been less than 
one year old (Fig. 22) , although we cannot exclude the possibility 
that these were young animals born out of season, in winter or 
spring. 

We often saw young animals (yearlings) accompanying females 
at sea, swimming beside them and touching noses with them. On 
15 July 1965 four female-yearling pairs were observed on land 
as part of an aggregation of animals which included no pups. One 
of the females moved away from the aggregation and the yearling 
followed, very much like a pup would. When the yearling crawled 
across the back of an adult, the two exchanged open-mouth threats, 
an interaction which was never seen between a pup and an adult. 
After moving 50 meters, the female lay down and the yearling began 
to suckle. 

Yearling members of such pairs are very active and independent. 
For example, they will temporarily leave the adult and rush to the 
sea during a minor disturbance. Even while suckling, they are 
aggressive and may nip the mother or threaten her; they change 
teats frequently and the intervals of suckling seem to be short. 
We saw female-yearling pairs in both breeding and non-breeding 
aggregations. Presumably the yearlings are not exclusively depen- 
dent on their mothers for nutrition, but we have no data on this 
subject. Nor do we know if a female with a new pup continues to 
maintain ties with the offspring born in previous years. 

Our observations on suckling in yearlings are not unique. Both 
Rowley (1929:18) and Orr (19656:19) have previously reported 
instances of young Zalophiis suckling long after they could reason- 
ably be classified as pups. 





Fig. 20. Pups play-fighting (top) . The pup on the right has just lunged 
forward in a manner similar to that of a fighting bull. 

Fig. 21. Female suckling pup on anterior teat (bottom) . 




Fig. 22. Female suckling a yearling 




qUAMTlFIABLE ASPECTS OF THE 
BIOLOGY OF ZALOPHUS 

Population Dynamics 

THE previous sections of this paper have 
developed a quahtative picture of the 
behavior of Zalophus and we can now at- 
tempt to quantify some aspects of its biol- 
ogy. 

Population Fluctuations.— The popula- 
tion of sea lions on San Nicolas Island dur- 
ing the breeding season remains in a state 
of flux. Individual adult males arrive from 
and depart for their non-breeding range at various times; pregnant 
females arrive at different times; some of the animals are always 
out at sea; bulls establish territories for a week or two then aban- 
don them; pups are added in large numbers; animals change from 
one rookery to another or shift between hauling ground and 
rookery. Consequently, population estimates based on numbers 
of animals on shore on a given day do little more than indicate 
a lower limit for the total population. 

The details of the distribution and numbers of Zalophus present 
on San Nicolas Island during the 1965 breeding season are shown 
in Table 1 and Figures 23 and 24. From May through September 
the population of adult females fluctuated between 2700 and 4200, 
but the number of territorial bulls never exceeded 408, and the 
number of non-breeders varied between 1600 and 2600. The maxi- 
mum number of pups were present on shore during early July and 
since their population at this time was a little over 3600, there were 
at least 3600 breeding females based on the island during the breed- 
ing season. The total population, excluding pups, varied between 
4100 and 5900. These last figures are more than twice those for the 
summer of 1949 (Bartholomew, 1951) but only slightly larger than 
those for the summer of 1958 (Bartholomew and Boolootian, 1960) . 

The relative changes in numbers in the different age and sex 
classes (Fig. 24) present on San Nicolas Island during the summer 

45 



46 The CaIifor7iia Sea Lion 



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Biology of Zalophus 47 

of 1965 are consistent with what is known of seasonal changes in 
behavior and distribution of the Zalophus of the coasts of California 
and Baja California. Most of the pups were born during May and 
June and the increase in numbers of pups during early summer was 
merely an expression of the rate at which they were being added to 
the population. During July the number of pups present on the 
island decreased, presumably because of deaths and because the 
pups were spending progressively more time at sea as they grew 
older. The increase in pups during September is probably too great 
to be accounted for by sampling error and may indicate that pups 
born on other islands had followed their mothers to San Nicolas. 
The Zalophus population on San Nicolas is larger in winter than 
in summer (Bartholomew and Boolootian, 1960:368) and it is 
reasonable to assume that the increase in all classes of animals, 
except territorial bulls, that occurred during late summer was a 
result of an influx of animals that were elsewhere during the breed- 
ing season. The decrease in the number of territorial bulls in late 
summer coincides with, and may be related to, the arrival in late 
July and early August of a large contingent of male Zalophus at 
Alio Nuevo Island, which lies 250 miles north of San Nicolas and 
which supports a very large population of male Zalophus during 
fall and winter (Orr and Poulter, 1965:394). 

The short-term fluctuations of the numbers of Zalophus on San 
Nicolas during the summer are relatively small and resemble those 
of Arctocephalus gazella (Paulian, 1964:34) more than those of 
Callorhinus (Peterson, 19656:20) . Perhaps this is because Zalophus 
and A. gazella feed closer to their breeding grounds than does 
Callorhinus. 

Changes in Local Distribution.— Although the size of the sea 
lion population on San Nicolas as a whole remained relatively 
stable from day to day, the number of sea lions on any one rookery 
showed erratic short-term variations. This is shown by the periodic 
censuses of the various rookeries (Table 1) and particularly by 
the daily censuses of the study area (Table 2) . Since we had many 
marked individuals in the study area we know that the fluctuations 
in animals there were caused by animals moving temporarily to 
other nearby areas as well as by their going to sea. It is obvious 
that because of the lack of close geographic ties, counts of a re- 



48 The California Sea Lion 



<i: 

_j 

CO 
CO 

<c 

_1 
o 
c_> 



CO 




Biology of Zalophus 49 

stricted area give little insight into the performance of the popula- 
tion of the island as a whole. 

Mortality on Land.— We found few carcasses of pups or adults 
on land at any season. Since Zalophus stay very close to the water 
during the breeding season, it is possible that some pup carcasses 
wash to sea soon after death, which complicates any estimate of mor- 
tality. However, the mortality rate of pups on land appeared to us 
to be much less than that of Callorhinus on the Pribilof Islands, 
which ranges between 5 and 16 per cent during the breeding season 
(North Pacific Fur Seal Commission, 1962:5) . For example, by 24 
July 1965 only ten dead pups had been observed during the previous 
six weeks in an area where approximately 250 pups had been born. 

On San Nicolas during the 1965 breeding season, natural mortal- 
ity of adults on land was negligible. Several of the carcasses of 
adults which appeared on the beaches of San Nicolas showed signs 
of human predation: buckshot wounds, or evidence of clubbing 
or stoning. 

Behavior of Individuals 

Territorial Bulls.— By using scars and wounds for identification, 
we were able to recognize and follow from day to day 14 individual 
bulls in the study area. Although we accumulated many partial 
records, we had difficulty in obtaining satisfactorily complete data 
on length of territory maintenance; scar patterns disappeared or 
were altered, and bulls remained partly submerged in the water 
during daylight, making marks difficult to see. Five reliable and 
complete records of duration of territory maintenance were ob- 
tained during June and July. The mean duration was 9 days (3, 9, 
9, 1 1, and 14 days) . Our other records, although incomplete, sug- 
gest that these data are typical. 

We have no evidence that individual bulls changed from one 
territory to another and we think that a given bull rarely assumes 
territorial status more than once during the season. However, in 
one instance a bull, marked by a projectile syringe in its flank, 
moved from a territory in Area IB to a hauling ground on July 14, 
and later came back to his original territory. 

Adult Females.— Despite a fairly good technique for marking 
females, we found difficulty in obtaining long serial records on them 



50 The California Sea Lion 

TABLE 2 
Population of Zalophus in Study Area During and After 1965 Breeding 

Season 



,-^ a 



^ c« 



p 




si 


a, 

3 
(1h 


s 


3 


3 


39 


40 


13.0 


5 


3 


33 


35 


11.0 


6 


4 


* 


29 




7 


4 


* 


28 




8 


3 


48 


33 


16.0 


9 


3 


51 


34 


17.0 


10 


3 


46 


32 


15.3 


11 


2 


47 


20 


23.5 


12 


2 


* 


26 




14 


2 


38 


20 


19.0 


15 


2 


* 


18 




16 


2 


* 


* 




17 


2 


20 


19 


10.0 


19 


4 


43 


48 


10.8 


20 


3 


37 


33 


12.3 


21 


4 


39 


40 


9.8 


22 


4 


31 


21 


7.8 


24 


4 


36 


30 


9.0 


27 


2 


40 


32 


20.0 


28 


2 


56 


37 


28.0 


29 


2 


52 


51 


26.0 



June 



July 



3 


3 


* 


30 




4 


3 


41 


44 


13.7 


5 


3 


38 


47 


12.7 


6 


3 


37 


44 


12.3 


7 


3 


43 


41 


14.3 


8 


3 


31 


36 


10.3 


9 


2 


* 


38 




12 


2 


29 


40 


14.5 


13 


2 


21 


29 


10.5 


14 


2 


28 


22 


14.0 


15 


1 


30 


25 


30.0 


17 


2 


38 


22 


19.0 


18 


2 


19 


23 


9.5 


20 


2 


15 


26 


7.5 


22 





20 


8 


- 


24 


2 


32 


20 


16.0 


27 


2 


19 


21 


9.5 


28 


1 


24 


9 


24.0 


29 


1 


25 


11 


25.0 



Aug. 



4 


1 


31 


12 


31.0 


5 


1 


37 


15 


37.0 


9 





12 


14 


- 


10 





26 


21 


- 


20 





19 


23 


- 


21 





7 


17 


- 


22 


1 


4 


10 


4.0 



Sept. 14 12 19 

15 1 21 26 10.5 

16 1 17 20 8.5 



Census incomplete. 



Biology of Zalophiis 51 



6000 r 




TERRITORIAL BULLS 



JUNE 



JULY 



AUGUST 



SEPT. 



Fig. 24. Zalophiis populations on San Nicolas Island during and after the 
1965 breeding season. See Table 1 for a more detailed analysis. 



because their extreme gregariousness made the marked animals hard 
to find and because they frequently changed locations in the 
rookery. Eight females with pups were marked and followed for 
approximately two months. To facilitate the assessment of the 
proportion of the time devoted to various activities, a marked fe- 
male was arbitrarily assumed to have been in residence for all of 
each day on which she was seen in the study area. 

The eight marked females were seen on shore in the study area an 
average of 34 per cent of the days under observation, and were seen 
suckling their pups on an average of 75 per cent of the days they 
were known to be on shore (Table 3) . However, the amount of 
time spent on shore and the number of days on which they were 
seen suckling changed as the season advanced. During mid-June, 
when the pups were new, the females were on land an average of 
50 per cent of the time, but thereafter the mean percentage de- 



52 The California Sea Lion 



TABLE 3 

Summary of Records for 8 Marked Females Over a Two-month Period 
During the 1965 Breeding Season 



Female 


Days 

of 
search 


Days 
ob- 
served 


Fraction of 
days present 
in study area 


Days 

observed 
suckling 


Fraction 
of days 
suckling 


A 


24 


9 


0.38 


3 


0.33 


B 


25 


10 


0.40 


9 


0.90 


C 


26 


12 


0.46 


11 


0.92 


D 


29 


11 


0.38 


10 


0.91 


E 


24 


11 


0.46 


10 


0.91 


F 


18 


5 


0.28 


3 


0.60 


G 


36 


5 


0.14 


3 


0.60 


H 


30 


6 


0.20 
X 0.34 


5 


0.83 
X 0.75 



creased markedly (Fig. 25) . The proportion of days on which they 
suckled their pups declined sharply during July (Fig. 26) . 

Parturition and Copulation.— From the population curves 
(Fig. 24) , it appears that most of the pups were born during June. 
However, females that were obviously pregnant were common at 
the study area from late May until about 10 Jvily. More accurate 
data on the temporal pattern of parturition will require observa- 
tions in April and May and more extensive population estimates. 

The first copulation we observed was on 16 June, and thereafter 
copulation became progressively more common until the second 
week of July. By 22 July, the number of sexually responsive females 
had declined sharply as had the number of territorial bulls. 

One marked female delivered during the night of 14 June and 
subsequently came into estrus 15 days later, on 29 June. To judge 
from this case, and the more general behavioral changes described 
above, we infer that estrus occurs approximately two weeks post 
partum. This period is similar to that in many other pinnipeds 
(Harrison et al., 1952:439) , but much longer than in Callorhinus 
(Bartholomew and Hoel, 1953:420). 

Pups.— The behavior of the 39 marked pups was classified in the 
same way as that of females. Six pups were observed on fewer than 
five days; 19 others were observed more often than this but were 
seen to be suckled on fewer than three days; while the other 14 were 



Biology of Zaloplius 53 



5 



80 



60 



40 



20 




FEMALES 



JUNE 



JULY 



AUGUST 



Fig. 25. Changes in the populations of 8 marked females and 39 pups in the 
study area. See also Tables 3 and 4. 



seen more than five days and were suckled on more than three. 
These figures suggest that fewer than half the pups caught and 
marked at one rookery were actually resident, in the sense of being 
permanently based there and being suckled there frequently. 

Excluding the group of six pups seen fewer than five times, on 
the average the marked pups were seen in the study area on 42 
per cent of the days, and on 22 per cent of the days on land they 
were suckled. Considering only the 14 resident individuals, the 
comparable figures are 45 and 34 per cent, respectively. 

The fraction of pups on land remained near the same level 
throughout the season (Fig. 25). Of the pups marked in early June, 
about one-third were observed in the study area on any subsequent 
day through early August. The percentage for resident pups was some- 
what higher than that of the non-residents. The fraction of the 
days on which the marked pups were seen being suckled decreased 
from a mean of 56 per cent in mid-June to about 15 per cent in 
late June and to almost zero in August (Fig. 26) . 

Patterns of Timing.— The data on time on land suggest that fe- 
males do not remain separate from their pups either for extended 
or regular periods. Comparison of records of one female and her 
marked, resident pup (Table 4) indicates synchronous timing of 



54 The California Sea Lion 



I 



100r 



FEMALES 




JUNE 



JULY 



AUGUST 



Fig. 26. Changes in amount of suckling by marked females and pups in the 
study area. See also Table 3. 



behavior. The pup was observed on more days than was its mother, 
but it did not remain in the study area rookery continually. The 
intervals between the female's visits to the rookery varied from 
one to four days with no regular periodicity. The same statements 
apply to other resident pups and to the adult females. 

In summary, a pup appears to be suckled at irregular intervals, 
which lengthen throughout the summer. Between times of suckling, 
both female and pup are concurrently absent from the rookery for 
considerable periods, particularly after the first ten days to two 
weeks post partum. The possibility exists that the pair may remain 
together elsewhere. This pattern contrasts sharply with the pro- 
longed suckling broken by long, regular intervals of separation, as 
found in Callorhinus (Bartholomew and Hoel, 1953:421) . 

Ratio of Breeding Females to Breeding Males.— The unex- 
pectedly low level of site tenacity in Zalophus makes it difficult 
to arrive at a satisfactory estimate of the effective sex ratio on the 
rookery. The aggregations of females are not harems in the sense 
of being groups controlled and maintained by the activities of the 



Biology of Zalophus 55 

TABLE 4 

Records of Occurrence of a Marked Female and Her Marked Pup in the 

Study Area on San Nicolas Island During Censuses on 28 Different Days 

Between 13 June and 7 August 1965 



Female seen 

Female seen with pup 

Female seen without pup 

Pup seen 

Pup seen without female 



)ays 


%o£ 

days of 

observation 


10 


36 


10 


36 








15 


54 


5 


18 



territorial bulls. The females and pups shift from territory to ter- 
ritory and may even alternate between different rookeries. The 
females spend part of their time at sea. All of the females are not 
in the same reproductive condition at a given time, nor do individ- 
ual bulls remain on territory throughout the breeding season. Con- 
sequently the raw census data shown in Tables 1 and 2 offer only 
a rough index to the effective sex ratio. 

The degree of polygyny in this species can be best appraised if 
we can estimate the mean number of estrous females available 
to each territorial bull. Unfortunately, because of factors listed 
above, we can hope to achieve no more than rough approximations 
of upper and lower limits for the number of females per territorial 
bull (Ftb) during the month when breeding was at its peak (15 
June to 15 July) . 

As shown in Table 1, at least 3604 pups were born on San Nicolas 
during the 1965 breeding season. This means that a minimum of 
3600 parous females must have come into estrous during the breed- 
ing season. To this figure we can add at least 500 as a conservative 
estimate of the number of nulliparous females coming into estrous. 
The population of territorial bulls on the island during the breed- 
ing season averaged about 320. If we were to assume that each ter- 
ritorial male remained on his territory for the full 30 days, Ftb = 

=13. However, we know from our study area data that the 

320 
males remained on territory for about 9 days. Therefore, approxi- 



56 The California Sea Lion 

mately 1060 of them would have occupied territories during the 

4100 
breeding season. Using these figures, F^b = — 4. From these 

computations we can conclude that on the island as a whole the 
ratio of estrous females to territorial bulls during the peak breeding 
month was at least 4 : 1 and could hardly have exceeded 13:1. 

As a check of the rough calculations based on the total Zalophus 
population on San Nicolas, we can consider the small population 
in the study area which was under almost continuous scrutiny and 
for which we have relatively detailed information. Crude estimates 
of Ftb in this small sample during the peak of the breeding season 
can be made if we assume that during this period (15 June to 15 
July) the rate at which females came into estrous remained constant 
and we know the following: 

a. The daily mean of the number of estrous females (14) . 

b. The mean fraction of time the females stayed on the rookery 
(0.3) . 

c. The length of the peak of the breeding season (30 days) . 

d. The mean duration of territory maintenance by bulls (9 
days) . 

e. The daily mean of the number of territorial bulls (3) . 

A consideration of the assumptions and data enumerated above 

shows that Ftb = ( — ) -^( — XeJ£s5, which is similar to the mini- 
mum possible figure obtained using the population as a whole. F^b 
can be calculated for the study area using the pups as an index to 
the number of estrous females. If we assume that the same number 
of pups entered the study area as left it, approximately 48 pups were 
born in the study area. If there were 48 pups, there were 48 parous 
estrous females, and we can add to these an estimate of 5 nulli- 
parous estrous females. If the mean number of bulls was 3 and 
the average stay on territory was 9 days, there were 10 bulls present 
during this period. Using these figures, F^b = 5 which approximates 
the minimum figure obtained from the population as a whole. 
Just for completeness we can examine the uncorrected data in 
Tables 1 and 2. During the breeding season the ratio of females to 
territorial bulls varied from rookery to rookery and from time to 
time. In the first week of June, the mean F^b for the island as a 



Biology of Zalophus 57 

whole was 7; dviring early July it was 12. On the study area during 
June and early July, F^b varied between 8 and 28, with a mean 
of 14. 

From the several approaches above, we can do no more than 
conclude that the ratio of estrous females to territorial bulls during 
the 1965 breeding season could hardly have been less than 4 : 1 
nor more than 14 : 1. 




DISCUSSIOJ^ 

ALTHOUGH otariids having the ma- 
jor morphological adaptations 
for aquatic life shown by living forms 
have been in existence since at least 
the upper middle Miocene (Mitchell, 
1962) , many features of the social be- 
havior of sea lions remain strikingly 
similar to those of terrestrial mam- 
mals. The particular pattern of social 
behavior shown by Zalophus appears 
to be determined by the interplay between gregariousness and ag- 
gressiveness on the one hand, and the behavioral and morphological 
adaptations associated with its amphibious mode of life on the other. 

Gregariousness 

Like other otariids, Zalophus are strongly gregarious when on 
land. Although the species has traditional hauling grounds which 
have been used for several decades at least, a local population may 
shift from one area to another during a single breeding season. 
Even with this mobility, Zalophus group themselves in crowded 
aggregations separated by extensive unoccupied areas. This sug- 
gests that the local crowding so characteristic of Zalophus rookeries 
and hauling grounds is not related to scarcity of suitable land 
habitat but rather to gregariousness. 

Terrestrial gregariousness in a wide-ranging marine mammal 
should help to assure that breeding groups adequate for reproduc- 
tion will occur. In the case of Zalophus, which is almost certainly 
monestrous and which has a brief period of estrus, access of the fe- 
male to a breeding bull during one limited period per year requires 
some special behavioral adaptations, an obvious one of which is 
gregariousness. 

Another obvious selective advantage favoring extreme gregarious- 
ness is that it increases the chance of detecting and escaping from 
terrestrial danger. The presence of large numbers of individuals, 

58 



Discussion 59 

any one of which by dashing toward the water can alert several 
hundred others to possible danger, compensates for the limited 
visual and olfactory acuity of the individual animals. 

Aggressiveness 

Gregariousness and aggressiveness are not mutually exclusive be- 
havioral attributes. Opportunities for intraspecific aggressive be- 
havior are obviously abundant in gregarious forms because of the 
frequency of contacts between individuals. In Zalophus, as in many 
other highly gregarious mammals, patterns of intraspecific aggres- 
sion are well developed and aggressive interactions contribute im- 
portantly to the social structure. Aggressive behavior is more de- 
veloped during the breeding season than at other times, and during 
the breeding season it is more intense on the rookeries than on the 
haviling grounds. In general, the aggressiveness of young animals 
is less than that of the adults. Breeding females react aggressively 
but with relatively low intensity toward other females, toward adult 
males, toward juveniles of both sexes, and often toward pups other 
than their own. The aggressive activities of breeding males are 
much more intense than those of the females, but are focused 
primarily on other males (including juveniles) . 

Like other mammals (Matthews, 1964:30), Zalophus shows a 
graded series of aggressive activities, extending from ritualized move- 
ments and threats to savage fighting. Even during their most violent 
fights, Zalophus bulls employ highly stereotyped movements and 
postures. Conseqviently the various customary categories of aggres- 
sive behavior are often difficult to distinguish. The classification 
which follows has a large arbitrary component. 

Threats.— Both postural and vocal threats are frequently used in 
encounters between males, females, and young, including pups. 
These threats vary in intensity from vocal signals indicating loca- 
tion or territory occupancy, to actions which are preliminary to 
violent fights. The basic threat is an open-mouthed thrust of the 
head accompanied by a vocalization, the characteristics of which 
vary with age and sex. This generalized expression of aggressive- 
ness is extremely common and may be used by disturbed adults, 
juveniles, or pups of either sex at any season and normally involves 
pairs of individuals. The most common cause of this threat is the 



60 The California Sea Lion 

disturbance caused by an animal moving into or through a crowded 
group. Usually the animal causing the disturbance merely moves 
aside or retreats a few feet when it is threatened, but it may threaten 
in return before continuing on its way. This threat usually marks 
the total extent of an aggressive encounter, but during the breeding 
season it is sometimes the prelude to a fight and in most cases an 
animal that threatens gives the impression of being prepared for 
such an eventuality. 

Fights.— Fights in which serious wounds are inflicted occur most 
often between males in relation to territorial establishment and 
maintenance. However, females that are immediately post partum 
may fight vigorously but they rarely inflict visible damage on each 
other. Interactions which employ many of the motor patterns of 
fighting are frequent among young non-breeding animals but these 
are of low intensity and appear to be play. 

Because they are so spectacular, the fights of otariid males have 
been cited as the most violent that occur among mammals. Never- 
theless, actual physical fighting occurs only under a limited set of 
circumstances. Zalophus bulls fight only when territory mainte- 
nance is involved. Bulls inflict nasty cuts and gashes, but we have 
never seen a fight result in death. Postural and vocal threats precede 
and are intermingled with attempts to inflict physical damage, and 
some of the motor patterns involved in actual fighting have become 
ritualized as described below. 

Ritualized Aggressive Behavior.— A significant component of 
the aggressive behavior of Zalophus bulls is ritualized in that it con- 
sists of signals which do not of themselves serve as threats. The rit- 
ualized aggressive behavior of Zalophus conforms to the original 
usage of Huxley (1923) in that its evolutionary derivation appears 
to be separate from its information content (cf. Lorenz, 1964:48) . 
This is shown with particvilar clarity by the stereotyped and 
formalized boundary ceremony which is frequently engaged in by 
bulls with adjacent territories. The vocalizing, head-shaking, neck- 
arching, falling-prostrate, and oblique-staring patterns of the cere- 
mony appear to be mutually recognized expressions of social statvxs 
and do not lead to fighting. Nevertheless, the individual elements 
of the ceremony resemble the motor patterns used during fights: (1) 
Head-shaking is much the same movement that territorial bulls use 
when violently seizing and shaking an opponent. (2) The falling- 



Discussion 61 

prostrate maneuver is very similar to the forward lunge with pro- 
tective appression of the foreflippers used by fighting territorial 
bulls. (3) The recurved-neck posture resembles a posture frequently 
seen during pauses between slashing bites. (4) The vocalizations 
used during ceremonies suggest prolonged versions of the grunts 
and gasps of fighting animals. 

In the absence of information on sea lions reared in isolation, 
we cannot say whether these ritualized patterns are genetic or 
learned, but the individual behavioral elements appear early in 
ontogeny. Whatever the mode of transmission and maintenance, 
the motor elements in the boundary ceremony closely resemble, 
and appear to have been derived from, behavior used in fighting, 
and represent examples of ritualized behavior in the sense of Huxley 
(but not necessarily in the sense of Blest, 1961) . 

Ontogeny of Aggressive Behavior.— The precociousness of sea 
lion pups, and the ease with which the behavior of animals of all 
ages and both sexes can be observed at hauling grounds, allows the 
ontogeny of aggressive behavior to be followed from infant to adult 
with unusual clarity. The mock-battles of pups employ motor pat- 
terns which clearly anticipate, in an unorganized way, modes of 
aggressive expression used by adults. The prolonged, energetic 
games of juveniles (aged one to five years) closely resemble the 
serious threatening and fighting of adults, but the patterns are 
still incompletely developed and appear unrelated to any imme- 
diate goal other than play. Adult males on hauling grounds interact 
almost exactly like territorial bulls, although less intensively. Thus, 
the maturing sea lion continually refines and standardizes the motor 
patterns of aggressive expression. 

Although these behavior patterns are performed frequently over 
a period of years, they are actually play until the individual be- 
comes an effective member of the breeding population. In sea 
lions as in the polecats [Miistela putorius) described by Poole 
(1966) , aggressive play consists of many activities that, in another 
context, are vital to reproduction and hence extremely important 
to survival. The transition from aggressive play to serious aggres- 
sion is extremely subtle. For example, a bull when moving into a 
rookery merely performs, with heightened intensity and persistence, 
behavior patterns which, only a short time before, he may have been 
performing playfully on the hauling ground. But as soon as he 



62 The California Sea Lion 

enters a rookery, these patterns of behavior determine whether or 
not he will establish a territory and become part of the breeding 
population. 

The motor patterns of play and serious aggression are so similar 
that in some circumstances we were unable to tell whether animals 
employing them were playing or not, and sometimes one member of 
a pair of participating animals may itself apparently briefly misin- 
terpret the situation; i.e., one member of an interacting pair reacts 
playfully but the other reacts in a seriously aggressive manner. It 
seems that even though the motor patterns involved in social inter- 
action appear to require several years of practice before they are 
performed accurately enough for them to have communicatory pre- 
cision, the information content of even an accurate motor perform- 
ance may be obscured except in the proper context. 

Effects of Aggressiveness on Social Organization.— In Zalo- 
phus the spatial organization and structure of the adult breeding 
population are determined by aggressive interactions between in- 
dividuals of the same sex. The interactions between females deter- 
mine their location and spacing; the interactions between males 
determine whether they can come ashore in the vicinity of the fe- 
males and also fix their location and spacing once they are ashore. 

Aggressive interactions also support the integrity of mother-young 
pairs. Females prevent their newborn pups from coming into con- 
tact with other females by maintaining territories around them- 
selves and by vigorously rebuffing any pup but their own which 
approaches. The pups at first appear to be attracted to any female, 
but the aggressiveness which they evoke from all females except 
their own mother strongly encourages them to react in a highly 
selective manner to the scores of females they encounter. This situ- 
ation must contribute to the rapid development of the mutual 
recognition of mother and pup which is a basic component of the 
social pattern in Zalophus. 

The extreme development of male secondary sexual characteris- 
tics in Zalophus is related to aggressiveness. The participation of 
a bull in reproductive activity depends upon his success in aggres- 
sive encounters with other bulls, since these encounters determine 
access to the females. Large size, thick neck pelage, and enlarged 
canines obviously enhance success in fighting, and so a condition 
exists which strongly favors the evolution and maintenance of 
these attributes (see Evolution of the Social System) . 



Discussion 63 

The territorial bulls show no aggressive reactions to pups, but 
their large size and rapid movements are an important source of 
potential danger to the pups. After they are a few weeks old the 
pups avoid active bulls. This has the effect of concentrating the 
pup pods in parts of the rookery where territorial activity is mini- 
mal and so contributes to the spatial organization of the rookeries. 

During the non-breeding season when aggressiveness is greatly 
reduced, spatial organization is almost completely random, and the 
aggregations have no stable structure. The low intensity, non- 
reproductive aggressive interactions which are so common on the 
hauling grounds appear to contribute to comfortable spacing. Sea 
lions tend to form closely packed groups, and mildly aggressive 
reactions simply keep the degree of crowding to acceptable levels. 

Suppression of Aggressive Behavior.— During the breeding sea- 
son, nearly all interactions between adult Zalophus have aggressive 
components and at least some of these must be suppressed for a 
stable social structure to persist, and for reproduction to be success- 
ful. Gregariousness balances the centrifvigal effects of aggressiveness, 
but for successful copulation or for suckling, individuals must be- 
come at least temporarily tolerant of, or submissive to, others. As 
would be expected, therefore, there are two circumstances in which 
the usually aggressive reactions of Zalophus females are clearly sup- 
pressed; when they seek out and suckle their own pups, and when 
they actively solicit the attentions of and then copulate with a 
sexually active male. 

Territorial bulls never suppress their aggressive behavior com- 
pletely. Even during courtship and copulation they often vigorously 
threaten and nip at their partners. The acceptance by bulls of ritu- 
alized signals in place of fighting at territorial boundaries may be 
an example of partial suppression of overt aggression. 



Amphibious Adaptations 

Although California sea lions spend much 
of their time on shore their adaptations for 
aquatic life profoundly influence their social 
behavior. They are essentially coastal animals. 
Large populations can be found on shore at 
all times of the year. During the breeding season they appear to 
have obligatory ties to land; there is no evidence that the young 




64 The California Sea Lion 

can be successfully born and suckled at sea. Copulation is usually 
terrestrial, but aquatic copulations occur with some frequency so 
this aspect of reproduction does not have to be terrestrial. 

Limited Terrestrial Mobility.— In pinnipeds the stability of 
the social structure during the breeding season appears to be strongly 
affected by their limited terrestrial mobility, which, together with 
their sedentariness, allows many highly aggressive males to remain 
physically close to each other without disruptive interactions. For 
example, in those groups of northern elephant seals, Mirounga an- 
gustirostris, which happen to breed on narrow beaches, the ordi- 
narily stable social structure often becomes chaotic at high tide 
when some of the females are partly awash (Bartholomew, 1952: 
415) . In this situation, the dominant male who is tied to the land 
cannot move rapidly enough to prevent the subordinate males that 
are swimming in the water from making rapid and repeated sorties 
into his harem. As a result the social structure temporarily disinte- 
grates into a confused melange of aggressive and sexual behavior. 

By analogy with this situation, it seems possible that the looseness 
of the social structure in Zalophus as compared with Callorhinus 
may be associated with the close association of its rookeries with 
water. For example, in Zalophus, but not in Callorhinus, the male 
territories nearly always abut on the water; the females remain near 
the water's edge; aquatic copulations are common; and aggressive 
interactions between territorial bulls and males in the water are so 
frequent that the social structure is subjected to repeated disruptive 
influences. 

Special Senses.— Pinnipeds are faced with the difficult problem 
of obtaining adequate sensory information from both air and water- 
media with very different physical qualities. Judging by behavioral 
criteria, visual acuity of Zalophus in air is limited both at close 
range and at a distance. As previously described, only large moving 
objects evoke responses and none of the social signals they employ 
requires precise visual discrimination. 

Although pinnipeds probably do not use olfactory clues under- 
water, during the terrestrial phases of their behavior Zalophus em- 
ploy olfaction in at least two circumstances; in mother-young recog- 
nition and in identification of estrous females by bulls. 

The marked development and conspicuous mobility of their 
vibrissae suggest that tactile information is important to Zalophus. 



Discussion 65 

Certainly this is the case in social interactions on land— nuzzling 
and nose rubbing are conspicuous in contacts between pairs or 
groups of individuals. 

Airborne acoustic signals are the primary means of social com- 
munication in Zalophus. We know of no other mammal which 
vocalizes as loudly and continuously as male Zalophus, and the fe- 
males and pups are almost as vocal as the males. By means of vocal 
signals bulls indicate their identity, status, location, and aggressive 
intent; females summon and recognize their pups and express their 
aggressiveness; pups reply to their mothers and signal to each other. 
The overwhelming emphasis on acoustic signals is probably related 
to the specialization of the other special senses for underwater func- 
tion. Vocal signaling also compensates in part for the limited ter- 
restrial mobility because it permits the animals to communicate 
over considerable distances. 

Thermoregulatory Requirements.— Because of their amphib- 
ious mode of life, pinnipeds are faced with conflicting thermoregu- 
latory needs. They must retain heat while in the water, but must 
be able to lose it while on land (Bartholomew and Wilke, 1956; 
Irving et al., 1962) . The conflicting demands for heat retention in 
water and heat dissipation in air are particularly acute for Zalophus 
because their warm-temperate and tropical distribution imposes a 
severe heat load on them when they are on shore. They have a low 
thermal conductance because of their large size and thick layer of 
subcutaneous fat. Consequently they face problems of overheating 
when they are active on land, especially in direct sunlight. They 
meet this probably in part by keeping wetted, thus taking advan- 
tage of evaporative cooling, and partly by staying on a wet sub- 
stratum to which they readily lose heat by conduction. It seems 
likely that the thermoregulatory necessity for staying wet has been 
an important selective factor favoring partly aquatic territories. As 
discussed above, this has in turn affected the entire pattern of the 
reproductive behavior in this genus. In addition, the high level 
of nocturnal activity in Zalophus may be related to the avoidance 
of overheating. 

Effects of Human Disturbance.— On the islands where they 
come ashore at the present time, Zalophus have no important ene- 
mies except man. Human disturbance has frequently driven other 
shore-breeding marine mammals nearly to extinction {Callorhinus, 



66 The California Sea Lion 

Enhydra, Mirounga, Arctocephalus) or caused them to abandon 
their hauHng grounds (for example, Eiimetopias in the Pribilofs, 
Kenyon, 1962) . Yet, some of the islands utilized by Zalophus have 
been occupied by Indians for several millenia (Meighan and Eber- 
hart, 1953; Hubbs and Roden, 1964), and occupation of them by 
man continues at the present time. 

Although in the Channel Islands Zaioplius appear to prefer 
windward shores with extensive rocky shelves, they also use sandy 
beaches, sheltered cobble beaches, and even talus at the base of 
rocky cliffs for rookeries. Furthermore, site tenacity in Zalophus is 
limited and contrasts sharply with the remarkably conservative pat- 
tern of Callorhinus (Peterson, 19656:93, 121) . Even the breeding 
population of Zalophus is mobile. If harassed by humans or dis- 
turbed by storms, scores of animals may abandon a beach and move 
to nearby areas and there reconstitute their social structure. Fe- 
males and pups move as pairs, by sea or overland, and a day or two 
after the females leave, the bulls abandon their territories. Other 
bulls, meanwhile, have established territories in the areas newly 
occupied by the females. These shifts may be local and temporary, 
or wide-spread and prolonged: we observed the temporary move- 
ment of one population as a result of our attempts to mark some 
individuals; and military operations have disturbed San Nicolas 
Area 1 so that during the past ten years the population there has 
gradually diminished despite a total increase in the island's popu- 
lation. 

This lack of restrictive site attachment appears to have contrib- 
uted to the success of Zalophus in the face of disturbance by mod- 
ern man, but we do not believe that Zalophus could withstand ex- 
cessive, widespread exploitation, or that incidental disturbance is 
harmless. The human popidation of California is competing in- 
creasingly with Zalophus, and protection of their rookeries will be 
necessary for continued survival of this valuable biological resource. 

Evolution of the Social System 

As Nutting (1891) pointed out, the breeding 
patterns of pinnipeds can be arranged in a series 
ranging from monogamous, relatively solitary 
genera with little sexual dimorphism (Phoca, 
Hydrurga) , to extremely polygynous, highly gre- 
garious, and dimorphic genera (Callorhinus, 
Arctocephalus) . Between these extremes are a 




Discussion 67 

variety of intermediates: semi-gregarious genera {Leptonychotes, 
Monachiis) , and others in which a harem system is partially de- 
veloped {Halichoerus, Cystophora) . The more polygynous species 
copulate on land while the more nearly monogamous ones do so in 
the water, and the breeding season tends to be more compressed 
in the polygynous forms (Bertram, 1940:127). The degree of ag- 
gressiveness between males, and gregariousness among females, gen- 
erally parallels the degree of polygyny. 

Sea lions (subfamily Otariinae) are generally assumed to belong 
at the highly polygynous end of this series. Possibly, this may be 
correct for Emnetopias (Mathisen et ah, 1962:470; Daetz, 1959) , for 
Otaria (Hamilton, 1934; vaz Ferreira, 1961) , and for the little- 
known Neophoca and Phocarctos (King, 1964) , although more 
adequate data are needed for all of these genera. Zalophus, however, 
can hardly be classified as one of the highly polygynous pinnipeds: 
copulation is frequently aquatic, females aggregate without regard 
to the territorial activity of bulls, male aggressiveness is relatively 
reduced and territories are held only briefly (actually less than the 
interval between parturition and estrous) , temporary residence in 
the rookery by non-breeding males is common, and the breeding 
season is relatively prolonged. Admittedly, sexual dimorphism is 
conspicuous and females far outnumber males on the rookeries (a 
point to which we shall return) . The social pattern of Zalophus 
resembles that of Halichoerus in the eastern Atlantic, which is 
rather loose and in which the degree of polygyny is moderate— both 
frequently copulate in the water, bulls cannot hold females against 
their will, male territories are short-lived and transient, and the 
females are relatively independent and aggressive (Hewer, 1960) . 
M^e suggest that these two species should be considered weakly 
polygynous and placed midway in the behavioral series. 

A reasonable hypothesis for the origin of polygynous social struc- 
ture in pinnipeds can be formulated from the ideas originated by 
Nutting (1891) and elaborated by Bertram (1940) and Bartholo- 
mew (1952) . The social pattern of the polygynous pinnipeds could 
have evolved as a result of the production of a combination of 
interactions between (1) their amphibious habits, (2) the gregar- 
iousness of the females, (3) the aggressiveness of males during 
the period of sexual activity, and (4) a basic pattern of reproduc- 
tive physiology similar to most mammals, which permitted the ex- 



68 The California Sea Lion 

elusion of some males from the breeding population. Breeding 
females tended to aggregate in crowded groups when on land while 
the males were attracted to the vicinity of the females but, being 
intolerant of each other, spaced themselves more widely. Thus, a 
local imbalance in the sex ratio developed. The competition be- 
tween males for location among the females led to this imbalance 
being self reinforcing. It also led to a selection for the epigamic 
and epideictic characters favoring success in aggressive interactions 
and elaborate patterns of threat behavior. This distortion of the 
effective sex ratio and exaggeration of sexual dimorphism could 
continue only until it became so extreme as to prevent an adequate 
reproductive rate. 

This model suggests a possible mechanism whereby the system 
may have evolved, but does not consider the functional advantages 
conferred by this system which must have favored its selection 
and maintenance over alternatives. W'ynne-Edwards (1962:525) 
has postulated that both gregarious breeding and polygamy are 
homeostatic adaptations by which regulation of population size 
is achieved and over-exploitation of the environment avoided. 
Wynne-Edwards' theory has been considered in relation to pinniped 
colonial behavior by Coulson and Hickling (1964:509) . We sug- 
gest that this polygynous breeding pattern may also have other 
selective advantages. For example, for a pinniped in which copula- 
tion is restricted to land where locomotion and communication are 
difficult, the polygynous system might well be advantageous simply 
as an efficient mechanism of re-uniting the sexes at the proper time 
each year. Thus, in a wide-ranging marine species of small total 
population size, maintenance of a minimum reprodtictive rate could 
be a problem, and colonialism and polygyny might have helped to 
obviate it. 

Our present concern is to evaluate these hypotheses and theories, 
and relate them to the observed patterns of behavior of Zalophus. Is 
the flexible polygynous system of Zalophus a stage in the gradual 
progression toward a more rigid pattern like that of Callorhinus} Or 
does it represent an evolutionary regression of the harem system, 
as the animals gradually become more aquatic in their reproduction? 

Bertram (1940:128), in reviewing the behavior of pinnipeds, 
has proposed that the relatively sketchy harem system in Halichoerus 
is vestigial, and that the ". . . truly polygynous habit, which in the 



Discussion 69 

past was the cause of the sexual disparity in size and behavior in 
this species, is now in the process of decay due to the growing inci- 
dence of aquatic copulation." The same logic is applicable to Zalo- 
phiis. The alternative, that polygyny and its concomitant features 
are just beginning to evolve, is less tenable since morphologic di- 
morphism is much more highly developed in Zalophus than seems 
warranted by the present, loose social pattern. 

Coulson (1965:55) objected to the idea that colonial breeding 
in grey seals is a vestigial behavior pattern, suggesting that if this 
behavior were not serving important functional purposes, it would 
rapidly disappear since "selection pressures against its persisting 
are high." We do not propose that all the selective advantages 
which once favored the evolution of colonial breeding, polygyny, 
and dimorphism have disappeared from the animals' present en- 
vironment. Instead, we suggest that some of the advantages, to 
Zalophus and Halichoerus at least, may have been reduced, result- 
ing in a partial regression of the harem system. An important factor 
contributing to the relaxing of the social structure may be the in- 
crease in frequency of aquatic copulation, which, as Bertram showed, 
could obviate much of the terrestrial social behavior of pinnipeds. 
Another potentially important factor for Zalophus is the relatively 
high temperature on their rookeries during the daytime, which 
apparently causes them to remain near the edge of the water. The 
high level of mobility of males in the water inakes prolonged main- 
tenance of territories abutting on the water difficult. (Even in Cal- 
lorhinus such territories are unstable, Bartholomew, 1953.) The 
frequent visits of females to the edge of the water for thermoregula- 
tory purposes impede the maintenance of stable harems and of 
course make possible aquatic copulations. It is noteworthy that in 
Callorhinus harem maintenance is relatively ineffective on the in- 
frequent sunny days on which heat stress causes females to leave the 
rookery and enter the water (Bartholomew and Hoel, 1953; Barthol- 
omew and Wilke, 1956) . 




SUMMARY 

THIS Study of the natural history and so- 
cial behavior of the California sea lion 
{Zalophus californianiis) is based primarily 
on observations made on San Nicolas Island, 
California, during the spring and summer of 
1965, but it draws on data acquired during 
the past two decades on the islands off Cali- 
fornia and Baja California and on the Galapagos Islands, Ecuador. 

Zalophus are highly gregarious at all seasons. They are primarily 
coastal in distribution and haul out on shore throughout the year. 
Although their primary locomotor adaptations are for swimming, 
with the primary propulsive force being supplied by simultaneous 
strokes of the front flippers, they are capable of effective locomotion 
on land. Their terrestrial locomotion is variable and includes 
walking, galloping, and striding with front flippers only. Despite 
their short pelage they groom themselves extensively using both 
front and hind flippers and occasionally the teeth. They also often 
rub themselves, particularly their snouts, against each other and 
against inanimate objects. They have no special postures for defe- 
cation or urination. 

While on shore they appear sensitive to high air temperatures and 
direct solar radiation and on warm days remain on the wet sand 
and rocks at the water's edge and frequently wet themselves down. 

The visual acuity of Zalophus in air seems limited, but they 
readily distinguish bold outlines and rapid movements. Auditory 
and olfactory discrimination appear to be well developed. 

Interspecific interactions with elephant seals, harbor seals, and a 
variety of birds are common. Zalophus tend to be dominant in their 
relations with harbor seals and young elephant seals, but not with 
adults of the latter. 

During the non-breeding season Zalophus appear to have no 
stable social organization while on shore although there are obvious, 
but presumably transient, size-related dominance relations. Except 
during the breeding season there is a strong, although incomplete, 
sexual allopatry. The adult males, but not the females or immature 

70 



Summary 71 

animals, tend to move to or beyond the northern part of the breed- 
ing range. 

During the non-breeding season the escape reaction of Zalophus 
is more pronounced than chiring the breeding season. At all seasons 
their alarm response is highly contagious and is often evoked by 
the alarm reaction of gulls associated with them. 

Zalophus are highly polygynous seasonal (June and July) 
breeders. Only territorial bulls participate in the breeding struc- 
ture. The male territories almost invariably abut on the water and 
some are completely aquatic. Bulls rarely hold territory continu- 
ously for more than two weeks so that a given territory may be oc- 
cupied by a succession of bulls during the breeding season. The 
males establish and maintain their territories by aggressive behavior 
involving almost incessant vocalization, frequent threats, and some- 
times violent fights. However, adjacent territorial bulls do not 
fight with each other, but maintain their status by stereotyped pat- 
terns of behavior apparently derived from the motor patterns of 
fighting. The females pay no attention to the boundaries of the 
male territories and move freely from territory to territory. The 
males do not attempt to hold the females so harems as such do not 
exist. 

Although the females are extremely gregarious they are suffi- 
ciently aggressive toward each other immediately post partum that 
for a few days they are almost territorial. The females are mon- 
estrous and estrus occurs about two weeks post partum. Copulation 
usually takes place on land but it often occurs partly or completely 
in the water. Females apparently usually copulate only once during 
estrus. 

Parturition usually occurs at night. The pups are extremely pre- 
cocious and can locomote and vocalize within a few minutes of 
birth. Females are very solicitous of their pups for the first few 
days and maintain almost continuous contact with them. They take 
their pups with them when changing positions in the rookery, and 
when going into the water to cool off they sometimes drag the 
pups with them into the water. As the season progresses the females 
spend progressively less time with their pups. 

By the time the pups are a couple of weeks old they aggregate into 
pods in the quieter parts of the rookery. They spend much time 
in play both on land and in the water. Their play anticipates many 



72 The California Sea Lion 

of the activities of the aduks, particularly fighting and copulation. 
They sjDcnd progressively more time in the water as they become 
older. 

Each female recognizes its own pup and suckles it to the exclusion 
of all others. Mother-pup recognition involves location by vocaliza- 
tion and final identification by olfaction. Visual clues appear to be 
of minor importance. Mother-young ties are sometimes maintained 
for a year or more and females are frequently seen nursing yearlings. 

From May through September 1965, the population of adult fe- 
males on San Nicolas Island fluctuated between 2700 and 4200, 
but the number of territorial bulls never exceeded 408. The total 
population on the island, excluding pups, fluctuated between 4100 
and 5900. Slightly more than 3600 pups were born. Pup mortality 
was very low compared with that of other highly colonial pinnipeds. 

The mean duration of territory maintenance of individually 
known bulls was 9 days and the longest was 14 days. The females 
appear to spend about one day in three ashore with their pups. 

Various methods of calculating the ratio of breeding females to 
breeding males on the rookeries yield figures that indicate that the 
ratio can hardly be less than 4 : 1 or more than 14 : 1. 

The particular pattern of social behavior shown by Zalophus 
appears to have been determined by the interplay between gregari- 
ousness and aggressiveness on the one hand and the behavioral and 
morphological adaptations associated with an amphibious mode of 
life on the other. The roles of each of these are discussed as are the 
relation of play to the ontogeny of aggressive behavior and factors 
contributing to the evolution and maintenance of the polygynous 
social structure of Zalophus. 



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76 The California Sea Lion 

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Index 



Aggressiveness: 

bulls, 23-25 

dimorphism related, 62 

effects, 62 

females, 29 

functions, 59 

ontogeny, 61 

ritualization, 60 

suppression, 63 

transition to play, 19, 61 
Alarm: 

call, 39 

learning, 20 

reaction, 17, 19 
Alio Nuevo Island, Calif., 47 
Aquatic copulation, 34, 68 
Arctocephalus gazella, 47, 66 

Bark: 

bull, 24 

female, 30 
Belch vocalization, female, 30 
Bertram, G. C. L., 67-68 
Birth, 31, 52 
Boundary ceremony, 25, 60, Figs. 

13-15 
Breeding sites, characteristics, 21, 66 
British Columbia, 4 
Bull, definition, 2 

California sea lion, see specific topic 
Callorhinus ursinus, comparison 

with Zalophus, see specific topic 
Carrying of pup, female, 36 
Chemical senses, 12 
Copulation, 33, 52, Figs. 17-18 
Corvus corax, 14 
Cough, 39 

Coulson, J. C, 68, 69 
Courtship, 32 
Crawl, 8 
Cystophora, 67 

Dates of breeding season, 52 
Delivery of fetus, 31 



Disturbance, see man 
Dominance relations: 

among females, 30 

among non-breeders, 18 

interspecific, 15 
Drinking, sea water, 9 

Eibl-Eibesfeldt, I., 1, 6, 27, 30, 40 

Elephant seal, see Mirounga 

Engraulis, 5 

Estrous display, 32 

Estrus, 32 

Eumetopias jubata, 14, 16, 67 

Excretion, 9 

Exploratory behavior, 39 

Female aggregations, 27 
Female:male ratio, 54-57 
Fighting: 

see also aggressiveness 

bulls, 23, 25, 60 

causes, 23 

females, 29, 60 

functions, 59 

length, 24 
Fiscus, C. H., 7 
Flipper-waving, 10 
Food, Zalophus, 5 
Fosterage, 41 

Fur seal, see Callorhinus or Arcto- 
cephalus 

Galapagos sea lion, see Zalophus 

californianus wollebaeki 
Gallop, 7, 37 
Gregariovisness: 

females, 28 

functions, 58 

non-breeders, 18 
Grooming, 8, Fig. 8 
Growl, 30 
Gull, see Larus 

Halichoerus, 41, 67-69 
Harbor seals, see Phoca 



77 



78 The California Sea Lion 



Harem: 

absence, 29 

system, 68 

use of word, 27 
Haviling ground, definition, 2 
Hearing, 12, 66 
Hickling, G., 68 
Huxley, J. S., 60 
Hydrurga, 66 

Immobilizing drugs and technique, 

4 
Individual recognition, see mater- 

nal-yoving recognition 
Intruders, territorial male, 25-26 

Jug posture, 6 

King, J. E., 13 

Land, time on, see periods of time 

on land 
Larus occidentalis, 14, 16 
Leptonychotes weddelli, 5, 67 
Lobodon carcinophagus, 8 

Man, response to, 11, 13, 17, 18, 20, 

21, 65-66, Fig. 9 
Marking sea lions for study, 3, Fig. 

2, Fig. 3, Fig. 4 
Maternal-young recognition, 41 

geography, 42 

olfaction, 43 

ontogeny, 41 

vision, 43 

vocalization, 42 
Maternal-young relations: 

first days, 36 

maintenance, 41 

post partum, 35 

seasonal change, 44 

separations, 37 

specificity, 41, Fig. 5 
Mazatlan, Mexico, 4 
Measurements, Zalophus, 4 
Merluccius, 5 
Migration, seasonal, 4, 47 
Mirounga angustirostris, 14, 15, 30, 
41,^64 



Model, evolutionary, 68 
Monachus, 14, 67 
Mortality, land, 49 
Mother-response call, 35, 38, 42 
Musky odor, bulls, 13 

Neck-rubbing, 33 

Neophoca, 67 

Nocturnal behavior, 11, 27, 28, 65 

Nursing, 37, 40, 67 

Nutting, C. C, 67 

Oblique stare, 25, Fig. 14 
Observations: 

methods, 2 

nocturnal, 3 
O'Gorman, F., 8 
Olfaction, 13, 42, 64 
Ontogeny, behavioral, 37 
Orr, R. T., 16, 44 
Otaria, 12, 67 

Paulian, P., 10 
Peace-keeping by bulls, 30 
Periods of time on land: 

bulls, 23, 49 

females, 49 

functions, 63 

pups, 52 

suckling, 54 

timing, 53 
Phalacorcorax pencillatus, 14 
Phocarctos, 67 
Phoca, 12, 14, 15, 66 
Play, 6, 19, 39, Figs. 19-20 

transition to fights, 61 

with inanimate objects, 40 
Pods, "pups, 38 
Population: 

fluctuations, 45 

estimates, San Nicolas I., 45, 47 
Porpoising, 6 

Polygyny, 21, 28, 54-57, 66-68 
Precociousness, pups, 35 
Pregnant females, 29, 31 
Pup-attraction call, 31, 35, 42 
Pup, definition, 2 



Index 



79 



Range, geograhic, Zalophus, 4 
Regurgitation, 9 
Ritualized behavior, 60 
Rookery: 

definition, 2 

determinants of location, 21, 66 
Rowley, J., 1, 44 

San Benitos, Islas, Mexico, 44 

San Nicolas I., Calif., 2, Fig. 1, 

Fig. 23 
San Miguel I., Calif., 4, 16 
Scammon, C. M., 1 
Sex ratio, 54-57 
Sexual segregation, seasonal, 4 
Shoreline territories, 22 
Site tenacity, 21, 66 
Sleeping postures, 8, 38, Figs. 6 

and 7 
Spatial distribution: 

breeding season, 22, 27, Fig. 12 

seasonal changes, 47 
Squeal, female, 30 
Steller, G. W., 31 
Steller sea lion, see Eumetopias 
Stride, 8 

Subordinate breeding males, 27 
Suckhng, 37, 40, 44, 54, Fig. 21 
Swimming, 6, 19 

copulation during, 34 

lessons, 37 

pups, 37 

Tactile sense, 13, 43 
Taste sense, 13, 43 
Terrestrial locomotion, 7, 64 
Territorial bull, definition, 2 
Territories of bulls, 22, Fig. 10 

aquatic, 22, Fig. 11 

boundaries, 22 

duration, 49 



establishment, 23 

intruders, 25 

maintenance, 25 

size, 22 
Thermoregulation, 10, 22, 28, 37, 

38, 65 
Threats: 

aggressive, 59 

play, 19 

open-mouthed, 29, 39, 41, 59, 
Fig. 16 
Thigmotaxis, 14, 18 
Tides, relation of rookery to, 22, 27 

Urocyon littoralis, 14 

Vibrissae, 13, 25 
Vision, 11, 26, 42, 64 
Vocalization: 

bulls, 24 

females, 30, 42 

nocturnal, 28 

ontogeny, 35 

pups, 38, 42 

ritualized, 61 

significance, 65 

Wandering bulls, 25 
Walk, 7, 37 

Weather, responses to, 9 
Weights, Zalophus, 4 
Whisker-stropping, 9 
Wind, responses to, 10, 21 
Wynne-Edwards, V. C, 68 

Yearling, behavior, 44, Fig. 22 

Zalophus californianus, see also spe- 
cific topic 
californianus, 4 
wollebaeki, 1, 4, 17, 40 
japonicus, 4 



Date Due 



ERNST MAYR LIBRARY 



3 2044 114 193 303 







9Br^